Re: [PATCH RFC] memory: pause all vCPUs for the duration of memory transactions

[Vitaly Kuznetsov]

David Hildenbrand <david@redhat.com> writes:

> On 26.10.20 11:43, David Hildenbrand wrote:
>> On 26.10.20 09:49, Vitaly Kuznetsov wrote:
>>> Currently, KVM doesn't provide an API to make atomic updates to memmap when
>>> the change touches more than one memory slot, e.g. in case we'd like to
>>> punch a hole in an existing slot.
>>>
>>> Reports are that multi-CPU Q35 VMs booted with OVMF sometimes print 
>>> something
>>> like
>>>
>>> !!!! X64 Exception Type - 0E(#PF - Page-Fault)  CPU Apic ID - 00000003 !!!!
>>> ExceptionData - 0000000000000010  I:1 R:0 U:0 W:0 P:0 PK:0 SS:0 SGX:0
>>> RIP  - 000000007E35FAB6, CS  - 0000000000000038, RFLAGS - 0000000000010006
>>> RAX  - 0000000000000000, RCX - 000000007E3598F2, RDX - 00000000078BFBFF
>>> ...
>>>
>>> The problem seems to be that TSEG manipulations on one vCPU are not atomic
>>> from other vCPUs views. In particular, here's the strace:
>>>
>>> Initial creation of the 'problematic' slot:
>>>
>>> 10085 ioctl(13, KVM_SET_USER_MEMORY_REGION, {slot=6, flags=0, 
>>> guest_phys_addr=0x100000,
>>>    memory_size=2146435072, userspace_addr=0x7fb89bf00000}) = 0
>>>
>>> ... and then the update (caused by e.g. mch_update_smram()) later:
>>>
>>> 10090 ioctl(13, KVM_SET_USER_MEMORY_REGION, {slot=6, flags=0, 
>>> guest_phys_addr=0x100000,
>>>    memory_size=0, userspace_addr=0x7fb89bf00000}) = 0
>>> 10090 ioctl(13, KVM_SET_USER_MEMORY_REGION, {slot=6, flags=0, 
>>> guest_phys_addr=0x100000,
>>>    memory_size=2129657856, userspace_addr=0x7fb89bf00000}) = 0
>>>
>>> In case KVM has to handle any event on a different vCPU in between these
>>> two calls the #PF will get triggered.
>>>
>>> An ideal solution to the problem would probably require KVM to provide a
>>> new API to do the whole transaction in one shot but as a band-aid we can
>>> just pause all vCPUs to make memory transations atomic.
>>>
>>> Reported-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
>>> Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
>>> ---
>>> RFC: Generally, memap updates happen only a few times during guest boot but
>>> I'm not sure there are no scenarios when pausing all vCPUs is undesireable
>>> from performance point of view. Also, I'm not sure if kvm_enabled() check
>>> is needed.
>>> Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
>>> ---
>>>  softmmu/memory.c | 11 +++++++++--
>>>  1 file changed, 9 insertions(+), 2 deletions(-)
>>>
>>> diff --git a/softmmu/memory.c b/softmmu/memory.c
>>> index fa280a19f7f7..0bf6f3f6d5dc 100644
>>> --- a/softmmu/memory.c
>>> +++ b/softmmu/memory.c
>>> @@ -28,6 +28,7 @@
>>>  
>>>  #include "exec/memory-internal.h"
>>>  #include "exec/ram_addr.h"
>>> +#include "sysemu/cpus.h"
>>>  #include "sysemu/kvm.h"
>>>  #include "sysemu/runstate.h"
>>>  #include "sysemu/tcg.h"
>>> @@ -1057,7 +1058,9 @@ static void 
>>> address_space_update_topology(AddressSpace *as)
>>>  void memory_region_transaction_begin(void)
>>>  {
>>>      qemu_flush_coalesced_mmio_buffer();
>>> -    ++memory_region_transaction_depth;
>>> +    if ((++memory_region_transaction_depth == 1) && kvm_enabled()) {
>>> +        pause_all_vcpus();
>>> +    }
>>>  }
>>>  
>>>  void memory_region_transaction_commit(void)
>>> @@ -1087,7 +1090,11 @@ void memory_region_transaction_commit(void)
>>>              }
>>>              ioeventfd_update_pending = false;
>>>          }
>>> -   }
>>> +
>>> +        if (kvm_enabled()) {
>>> +            resume_all_vcpus();
>>> +        }
>>> +    }
>>>  }
>>>  
>>>  static void memory_region_destructor_none(MemoryRegion *mr)
>>>
>> 
>> This is in general unsafe. pause_all_vcpus() will temporarily drop the
>> BQL, resulting in bad things happening to caller sites.

Oh, I see, thanks! I was expecting there's a reason we don't have this
simple fix in already :-)

>> 
>> I studies the involved issues quite intensively when wanting to resize
>> memory regions from virtio-mem code. It's not that easy.
>> 
>> Have a look at my RFC for resizing. You can apply something similar to
>> other operations.
>> 
>> https://www.mail-archive.com/qemu-devel@nongnu.org/msg684979.html
>
> Oh, and I even mentioned the case you try to fix here back then
>
> "
> Instead of inhibiting during the region_resize(), we could inhibit for the
> hole memory transaction (from begin() to commit()). This could be nice,
> because also splitting of memory regions would be atomic (I remember there
> was a BUG report regarding that), however, I am not sure if that might
> impact any RT users.
> "
>
> The current patches live in
> https://github.com/davidhildenbrand/qemu/commits/virtio-mem-next
>
> Especially
>
> https://github.com/davidhildenbrand/qemu/commit/433fbb3abed20f15030e42f2b2bea7e6b9a15180
>
>

I'm not sure why we're focusing on ioctls here. I was debugging my case
quite some time ago but from what I remember it had nothing to do with
ioctls from QEMU. When we are removing a memslot any exit to KVM may
trigger an error condition as we'll see that vCPU or some of our
internal structures (e.g. VMCS for a nested guest) references
non-existent memory. I don't see a good solution other than making the
update fully atomic from *all* vCPUs point of view and this requires
stopping all CPUs -- either from QEMU or from KVM.

Resizing a slot can probably be done without removing it first, however,
I expect that organizing QEMU code in a way where it will decide whether
or not old configuration requires removal is not easy. In some cases
(e.g. punching a KVM_MEM_READONLY hole in the middle of an RW slot) it
seems to be impossible to do with current KVM API.


> I haven't proceeded in upstreaming because I'm still busy with
> virtio-mem thingies in the kernel.


-- 
Vitaly