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

[Laszlo Ersek <lersek@redhat.com>]

On 11/03/20 14:07, Vitaly Kuznetsov wrote:
> Peter Xu <peterx@redhat.com> writes:
> 
>> Vitaly,
>>
>> On Mon, Oct 26, 2020 at 09:49:16AM +0100, 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.
>>
>> A pure question: Why a #PF?  Is it injected into the guest?
>>
> 
> Yes, we see a #PF injected in the guest during OVMF boot.
> 
>> My understanding (which could be wrong) is that all thing should start with a
>> vcpu page fault onto the removed range, then when kvm finds that the memory
>> accessed is not within a valid memslot (since we're adding it back but not
>> yet), it'll become an user exit back to QEMU assuming it's an MMIO access.  Or
>> am I wrong somewhere?
> 
> In case it is a normal access from the guest, yes, but AFAIR here
> guest's CR3 is pointing to non existent memory and when KVM detects that
> it injects #PF by itself without a loop through userspace.
> 

Indeed that's how I seem to remember it too; the guest page tables
cannot be walked (by the processor implicitly, or by KVM explicitly -- I
can't tell which one of those applies).

Thanks
Laszlo