Re: [Qemu-devel] How to reserve guest physical region for ACPI

[Igor Mammedov]

On Wed, 30 Dec 2015 21:52:32 +0200
"Michael S. Tsirkin" <address@hidden> wrote:

> On Wed, Dec 30, 2015 at 04:55:54PM +0100, Igor Mammedov wrote:
> > On Mon, 28 Dec 2015 14:50:15 +0200
> > "Michael S. Tsirkin" <address@hidden> wrote:
> >   
> > > On Mon, Dec 28, 2015 at 10:39:04AM +0800, Xiao Guangrong wrote:  
> > > > 
> > > > Hi Michael, Paolo,
> > > > 
> > > > Now it is the time to return to the challenge that how to reserve guest
> > > > physical region internally used by ACPI.
> > > > 
> > > > Igor suggested that:
> > > > | An alternative place to allocate reserve from could be high memory.
> > > > | For pc we have "reserved-memory-end" which currently makes sure
> > > > | that hotpluggable memory range isn't used by firmware
> > > > (https://lists.nongnu.org/archive/html/qemu-devel/2015-11/msg00926.html)
> > > >   
> > > 
> > > I don't want to tie things to reserved-memory-end because this
> > > does not scale: next time we need to reserve memory,
> > > we'll need to find yet another way to figure out what is where.  
> > Could you elaborate a bit more on a problem you're seeing?
> > 
> > To me it looks like it scales rather well.
> > For example lets imagine that we adding a device
> > that has some on device memory that should be mapped into GPA
> > code to do so would look like:
> > 
> >   pc_machine_device_plug_cb(dev)
> >   {
> >    ...
> >    if (dev == OUR_NEW_DEVICE_TYPE) {
> >        memory_region_add_subregion(as, current_reserved_end, &dev->mr);
> >        set_new_reserved_end(current_reserved_end + 
> > memory_region_size(&dev->mr));
> >    }
> >   }
> > 
> > we can practically add any number of new devices that way.  
> 
> Yes but we'll have to build a host side allocator for these, and that's
> nasty. We'll also have to maintain these addresses indefinitely (at
> least per machine version) as they are guest visible.
> Not only that, there's no way for guest to know if we move things
> around, so basically we'll never be able to change addresses.
simplistic GPA allocator in snippet above does the job,

if one unconditionally adds a device in new version then yes
code has to have compat code based on machine version.
But that applies to any device that gas a state to migrate
or to any address space layout change.

However device that directly maps addresses doesn't have to
have fixed address though, it could behave the same way as
PCI device with BARs, with only difference that its
MemoryRegions are mapped before guest is running vs
BARs mapped by BIOS.
It could be worth to create a generic base device class
that would do above. Then it could be inherited from and
extended by concrete device implementations.

> >    
> > > I would like ./hw/acpi/bios-linker-loader.c interface to be extended to
> > > support 64 bit RAM instead (and maybe a way to allocate and
> > > zero-initialize buffer without loading it through fwcfg), this way bios
> > > does the allocation, and addresses can be patched into acpi.  
> > and then guest side needs to parse/execute some AML that would
> > initialize QEMU side so it would know where to write data.  
> 
> Well not really - we can put it in a data table, by itself
> so it's easy to find.
> 
> AML is only needed if access from ACPI is desired.
in both cases (VMGEN, NVDIMM) access from ACPI is required
as minimum to write address back to QEMU and for NVDIM
to pass _DSM method data between guest and QEMU.

> 
> 
> > bios-linker-loader is a great interface for initializing some
> > guest owned data and linking it together but I think it adds
> > unnecessary complexity and is misused if it's used to handle
> > device owned data/on device memory in this and VMGID cases.  
> 
> I want a generic interface for guest to enumerate these things.  linker
> seems quite reasonable but if you see a reason why it won't do, or want
> to propose a better interface, fine.
> 
> PCI would do, too - though windows guys had concerns about
> returning PCI BARs from ACPI.
There were potential issues with pSeries bootloader that treated
PCI_CLASS_MEMORY_RAM as conventional RAM but it was fixed.
Could you point out to discussion about windows issues?

What VMGEN patches that used PCI for mapping purposes were
stuck at, was that it was suggested to use PCI_CLASS_MEMORY_RAM
class id but we couldn't agree on it.

VMGEN v13 with full discussion is here
https://patchwork.ozlabs.org/patch/443554/
So to continue with this route we would need to pick some other
driver less class id so windows won't prompt for driver or
maybe supply our own driver stub to guarantee that no one
would touch it. Any suggestions?

> 
> 
> > There was RFC on list to make BIOS boot from NVDIMM already
> > doing some ACPI table lookup/parsing. Now if they were forced
> > to also parse and execute AML to initialize QEMU with guest
> > allocated address that would complicate them quite a bit.  
> 
> If they just need to find a table by name, it won't be
> too bad, would it?
that's what they were doing scanning memory for static NVDIMM table.
However if it were DataTable, BIOS side would have to execute
AML so that the table address could be told to QEMU.

In case of direct mapping or PCI BAR there is no need to initialize
QEMU side from AML.
That also saves us IO port where this address should be written
if bios-linker-loader approach is used.

> 
> > While with NVDIMM control memory region mapped directly by QEMU,
> > respective patches don't need in any way to initialize QEMU,
> > all they would need just read necessary data from control region.
> > 
> > Also using bios-linker-loader takes away some usable RAM
> > from guest and in the end that doesn't scale,
> > the more devices I add the less usable RAM is left for guest OS
> > while all the device needs is a piece of GPA address space
> > that would belong to it.  
> 
> I don't get this comment. I don't think it's MMIO that is wanted.
> If it's backed by qemu virtual memory then it's RAM.
Then why don't allocate video card VRAM the same way and try to explain
user that a guest started with '-m 128 -device cirrus-vga,vgamem_mb=64Mb'
only has 64Mb of available RAM because of we think that on device VRAM
is also RAM.

Maybe I've used MMIO term wrongly here but it roughly reflects the idea
that on device memory (whether it's VRAM, NVDIMM control block or VMGEN
area) is not allocated from guest's usable RAM (as described in E820)
but rather directly mapped in guest's GPA and doesn't consume available
RAM as guest sees it. That's also the way it's done on real hardware.

What we need in case of VMGEN ID and NVDIMM is on device memory
that could be directly accessed by guest.
Both direct mapping or PCI BAR do that job and we could use simple
static AML without any patching.

> > > 
> > > See patch at the bottom that might be handy.
> > >   
> > > > he also innovated a way to use 64-bit address in DSDT/SSDT.rev = 1:
> > > > | when writing ASL one shall make sure that only XP supported
> > > > | features are in global scope, which is evaluated when tables
> > > > | are loaded and features of rev2 and higher are inside methods.
> > > > | That way XP doesn't crash as far as it doesn't evaluate unsupported
> > > > | opcode and one can guard those opcodes checking _REV object if 
> > > > neccesary.
> > > > (https://lists.nongnu.org/archive/html/qemu-devel/2015-11/msg01010.html)
> > > >   
> > > 
> > > Yes, this technique works.
> > > 
> > > An alternative is to add an XSDT, XP ignores that.
> > > XSDT at the moment breaks OVMF (because it loads both
> > > the RSDT and the XSDT, which is wrong), but I think
> > > Laszlo was working on a fix for that.  
> > Using XSDT would increase ACPI tables occupied RAM
> > as it would duplicate DSDT + non XP supported AML
> > at global namespace.  
> 
> Not at all - I posted patches linking to same
> tables from both RSDT and XSDT at some point.
> Only the list of pointers would be different.
if you put XP incompatible AML in separate SSDT and link it
only from XSDT than that would work but if incompatibility
is in DSDT, one would have to provide compat DSDT for RSDT
an incompat DSDT for XSDT.

So far policy was don't try to run guest OS on QEMU
configuration that isn't supported by it.
For example we use VAR_PACKAGE when running with more
than 255 VCPUs (commit b4f4d5481) which BSODs XP.

So we can continue with that policy with out resorting to
using both RSDT and XSDT,
It would be even easier as all AML would be dynamically
generated and DSDT would only contain AML elements for
a concrete QEMU configuration.

> > So far we've managed keep DSDT compatible with XP while
> > introducing features from v2 and higher ACPI revisions as
> > AML that is only evaluated on demand.
> > We can continue doing so unless we have to unconditionally
> > add incompatible AML at global scope.
> >   
> 
> Yes.
> 
> > >   
> > > > Michael, Paolo, what do you think about these ideas?
> > > > 
> > > > Thanks!  
> > > 
> > > 
> > > 
> > > So using a patch below, we can add Name(PQRS, 0x0) at the top of the
> > > SSDT (or bottom, or add a separate SSDT just for that).  It returns the
> > > current offset so we can add that to the linker.
> > > 
> > > Won't work if you append the Name to the Aml structure (these can be
> > > nested to arbitrary depth using aml_append), so using plain GArray for
> > > this API makes sense to me.
> > >   
> > > --->  
> > > 
> > > acpi: add build_append_named_dword, returning an offset in buffer
> > > 
> > > This is a very limited form of support for runtime patching -
> > > similar in functionality to what we can do with ACPI_EXTRACT
> > > macros in python, but implemented in C.
> > > 
> > > This is to allow ACPI code direct access to data tables -
> > > which is exactly what DataTableRegion is there for, except
> > > no known windows release so far implements DataTableRegion.  
> > unsupported means Windows will BSOD, so it's practically
> > unusable unless MS will patch currently existing Windows
> > versions.  
> 
> Yes. That's why my patch allows patching SSDT without using
> DataTableRegion.
> 
> > Another thing about DataTableRegion is that ACPI tables are
> > supposed to have static content which matches checksum in
> > table the header while you are trying to use it for dynamic
> > data. It would be cleaner/more compatible to teach
> > bios-linker-loader to just allocate memory and patch AML
> > with the allocated address.  
> 
> Yes - if address is static, you need to put it outside
> the table. Can come right before or right after this.
> 
> > Also if OperationRegion() is used, then one has to patch
> > DefOpRegion directly as RegionOffset must be Integer,
> > using variable names is not permitted there.  
> 
> I am not sure the comment was understood correctly.
> The comment says really "we can't use DataTableRegion
> so here is an alternative".
so how are you going to access data at which patched
NameString point to?
for that you'd need a normal patched OperationRegion
as well since DataTableRegion isn't usable.

> 
> >   
> > > 
> > > Signed-off-by: Michael S. Tsirkin <address@hidden>
> > > 
> > > ---
> > > 
> > > diff --git a/include/hw/acpi/aml-build.h b/include/hw/acpi/aml-build.h
> > > index 1b632dc..f8998ea 100644
> > > --- a/include/hw/acpi/aml-build.h
> > > +++ b/include/hw/acpi/aml-build.h
> > > @@ -286,4 +286,7 @@ void acpi_build_tables_cleanup(AcpiBuildTables 
> > > *tables, bool mfre);
> > >  void
> > >  build_rsdt(GArray *table_data, GArray *linker, GArray *table_offsets);
> > >  
> > > +int
> > > +build_append_named_dword(GArray *array, const char *name_format, ...);
> > > +
> > >  #endif
> > > diff --git a/hw/acpi/aml-build.c b/hw/acpi/aml-build.c
> > > index 0d4b324..7f9fa65 100644
> > > --- a/hw/acpi/aml-build.c
> > > +++ b/hw/acpi/aml-build.c
> > > @@ -262,6 +262,32 @@ static void build_append_int(GArray *table, uint64_t 
> > > value)
> > >      }
> > >  }
> > >  
> > > +/* Build NAME(XXXX, 0x0) where 0x0 is encoded as a qword,
> > > + * and return the offset to 0x0 for runtime patching.
> > > + *
> > > + * Warning: runtime patching is best avoided. Only use this as
> > > + * a replacement for DataTableRegion (for guests that don't
> > > + * support it).
> > > + */
> > > +int
> > > +build_append_named_qword(GArray *array, const char *name_format, ...)
> > > +{
> > > +    int offset;
> > > +    va_list ap;
> > > +
> > > +    va_start(ap, name_format);
> > > +    build_append_namestringv(array, name_format, ap);
> > > +    va_end(ap);
> > > +
> > > +    build_append_byte(array, 0x0E); /* QWordPrefix */
> > > +
> > > +    offset = array->len;
> > > +    build_append_int_noprefix(array, 0x0, 8);
> > > +    assert(array->len == offset + 8);
> > > +
> > > +    return offset;
> > > +}
> > > +
> > >  static GPtrArray *alloc_list;
> > >  
> > >  static Aml *aml_alloc(void)
> > > 
> > >   
> --
> To unsubscribe from this list: send the line "unsubscribe kvm" in
> the body of a message to address@hidden
> More majordomo info at  http://vger.kernel.org/majordomo-info.html