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

[David Hildenbrand]

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

-- 
Thanks,

David / dhildenb