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

[David Hildenbrand <david@redhat.com>]

On 27.10.20 14:02, Vitaly Kuznetsov wrote:
> David Hildenbrand <david@redhat.com> writes:
> 
>> On 27.10.20 13:36, Vitaly Kuznetsov wrote:
>>> 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.
>>
>> I cannot follow. My patch waits until *any* KVM ioctls are out of the
>> kernel. That includes VCPUs, but also other ioctls (because there are
>> some that require a consistent memory block state).
>>
>> So from a KVM point of view, the CPUs are stopped.
> 
> Sorry for not being clear: your patch looks good to me, what I tried to
> say is that with the current KVM API the only way to guarantee atomicity
> of the update is to make vCPUs stop (one way or another), kicking them
> out and preventing new IOCTLs from being dispatched is one way
> (temporary pausing them inside KVM would be another, for example -- but
> that would require *new* API supplying the whole transaction and not one
> memslot update).

Ah, got it.

Yes - and I briefly looked into resizing slots inside KVM atomically and 
it already turned out to be a major pain. All that metadata that's 
allocated for a memory slot based on the size is problematic.

Same applies to all other kinds of operations (splitting, punching out, 
...) as you also mentioned.

-- 
Thanks,

David / dhildenb