Re: Why does the vmovdqu works for passthrough device but crashes for emulated device with "illegal operand" error (in x86_64 QEMU, -accel = kvm) ?

[Paolo Bonzini]

On 3/4/24 22:59, Alex Williamson wrote:
> Since you're not seeing a KVM_EXIT_MMIO I'd guess this is more of a KVM
> issue than QEMU (Cc kvm list).  Possibly KVM doesn't emulate vmovdqu
> relative to an MMIO access, but honestly I'm not positive that AVX
> instructions are meant to work on MMIO space.  I'll let x86 KVM experts
> more familiar with specific opcode semantics weigh in on that.

Indeed, KVM's instruction emulator supports some SSE MOV instructions 
but not the corresponding AVX instructions.

Vector instructions however do work on MMIO space, and they are used 
occasionally especially in combination with write-combining memory.  SSE 
support was added to KVM because some operating systems used SSE 
instructions to read and write to VRAM.  However, so far we've never 
received any reports of OSes using AVX instructions on devices that QEMU 
can emulate (as opposed to, for example, GPU VRAM that is passed through).

Thanks,

Paolo

> Is your "program" just doing a memcpy() with an mmap() of the PCI BAR
> acquired through pci-sysfs or a userspace vfio-pci driver within the
> guest?
>
> In QEMU 4a2e242bbb30 ("memory: Don't use memcpy for ram_device
> regions") we resolved an issue[1] where QEMU itself was doing a memcpy()
> to assigned device MMIO space resulting in breaking functionality of
> the device.  IIRC memcpy() was using an SSE instruction that didn't
> fault, but didn't work correctly relative to MMIO space either.
>
> So I also wouldn't rule out that the program isn't inherently
> misbehaving by using memcpy() and thereby ignoring the nature of the
> device MMIO access semantics.  Thanks,
>
> Alex
>
> [1]https://bugs.launchpad.net/qemu/+bug/1384892