Re: [PATCH v3] memory: Directly dispatch alias accesses on origin memory region

[Mark Cave-Ayland]

On 19/04/2021 21:58, Philippe Mathieu-Daudé wrote:

> Hi Mark,
>
> On 4/19/21 10:13 PM, Mark Cave-Ayland wrote:
> > On 17/04/2021 15:02, Philippe Mathieu-Daudé wrote:
> >
> > > Since commit 2cdfcf272d ("memory: assign MemoryRegionOps to all
> > > regions"), all newly created regions are assigned with
> > > unassigned_mem_ops (which might be then overwritten).
> > >
> > > When using aliased container regions, and there is no region mapped
> > > at address 0 in the container, the memory_region_dispatch_read()
> > > and memory_region_dispatch_write() calls incorrectly return the
> > > container unassigned_mem_ops, because the alias offset is not used.
> > >
> > > The memory_region_init_alias() flow is:
> > >
> > >     memory_region_init_alias()
> > >     -> memory_region_init()
> > >        -> object_initialize(TYPE_MEMORY_REGION)
> > >           -> memory_region_initfn()
> > >              -> mr->ops = &unassigned_mem_ops;
> > >
> > > Later when accessing the alias, the memory_region_dispatch_read()
> > > flow is:
> > >
> > >     memory_region_dispatch_read(offset)
> > >     -> memory_region_access_valid(mr)   <- offset is ignored
> > >        -> mr->ops->valid.accepts()
> > >           -> unassigned_mem_accepts()
> > >           <- false
> > >        <- false
> > >      <- MEMTX_DECODE_ERROR
> > >
> > > The caller gets a MEMTX_DECODE_ERROR while the access is OK.
> > >
> > > Fix by dispatching aliases recusirvely, accessing its origin region
> > > after adding the alias offset.
> > >
> > > Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
> > > ---
> > > v3:
> > > - reworded, mentioning the "alias to container" case
> > > - use recursive call instead of while(), because easier when debugging
> > >     therefore reset Richard R-b tag.
> > > v2:
> > > - use while()
> > > ---
> > >    softmmu/memory.c | 10 ++++++++++
> > >    1 file changed, 10 insertions(+)
> > >
> > > diff --git a/softmmu/memory.c b/softmmu/memory.c
> > > index d4493ef9e43..23bdbfac079 100644
> > > --- a/softmmu/memory.c
> > > +++ b/softmmu/memory.c
> > > @@ -1442,6 +1442,11 @@ MemTxResult
> > > memory_region_dispatch_read(MemoryRegion *mr,
> > >        unsigned size = memop_size(op);
> > >        MemTxResult r;
> > >    +    if (mr->alias) {
> > > +        return memory_region_dispatch_read(mr->alias,
> > > +                                           addr + mr->alias_offset,
> > > +                                           pval, op, attrs);
> > > +    }
> > >        if (!memory_region_access_valid(mr, addr, size, false, attrs)) {
> > >            *pval = unassigned_mem_read(mr, addr, size);
> > >            return MEMTX_DECODE_ERROR;
> > > @@ -1486,6 +1491,11 @@ MemTxResult
> > > memory_region_dispatch_write(MemoryRegion *mr,
> > >    {
> > >        unsigned size = memop_size(op);
> > >    +    if (mr->alias) {
> > > +        return memory_region_dispatch_write(mr->alias,
> > > +                                            addr + mr->alias_offset,
> > > +                                            data, op, attrs);
> > > +    }
> > >        if (!memory_region_access_valid(mr, addr, size, true, attrs)) {
> > >            unassigned_mem_write(mr, addr, data, size);
> > >            return MEMTX_DECODE_ERROR;
> >
> > Whilst working on my q800 patches I realised that this was a similar
> > problem to the one I had with my macio.alias implementation at [1]:
> > except that in my case the unassigned_mem_ops mr->ops->valid.accepts()
> > function was being invoked on a ROM memory region instead of an alias. I
> > think this is exactly the same issue that you are attempting to fix with
> > your related patch at
> > https://lists.gnu.org/archive/html/qemu-devel/2021-04/msg03190.html
> > ("memory: Initialize MemoryRegionOps for RAM memory regions").
>
> So if 2 contributors hit similar issues, there is something wrong with
> the API. I don't see your use case or mine as forbidded by the
> documentation in "exec/memory.h".
>
> My patch might not be the proper fix, but we need to figure out how
> to avoid others to hit the same problem, as it is very hard to debug.

I agree with this sentiment: it has taken me a while to figure out what was 
happening, and that was only because I spotted accesses being rejected with -d 
guest_errors.

From my perspective the names memory_region_dispatch_read() and 
memory_region_dispatch_write() were the misleading part, although I remember thinking 
it odd whilst trying to use them that I had to start delving into sections etc. just 
to recurse a memory access.

> At least an assertion and a comment.
>
> > I eventually realised that I needed functions that could dispatch
> > reads/writes to both IO memory regions and ROM memory regions, and that
> > functionality is covered by the address_space_*() access functions.
> > Using the address_space_*() functions I was then able to come up with
> > the working implementation at [2] that handles accesses to both IO
> > memory regions and ROM memory regions correctly.
> >
> > The reason I initially used the
> > memory_region_dispatch_read()/memory_region_dispatch_write() functions
> > was because I could see that was how the virtio devices dispatched
> > accesses through the proxy. However I'm wondering now if this API can
> > only be used for terminating IO memory regions, and so the alias_offset
> > that you're applying above should actually be applied elsewhere instead.
>
> I figured out the AddressSpace API make these cases simpler, but IIRC
> there is some overhead, some function tries to resolve / update
> something and iterate over a list. While from the MemoryRegion API we
> already know which region we want to access.
>
> I Cc'ed Peter considering him expert in this area, but don't know else
> who to ask for help on this topic...

Yeah possibly Paolo, otherwise it's a dig through the git history of memory.c :/


ATB,

Mark.