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

[Peter Xu]

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?

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?

Thanks,

-- 
Peter Xu