Hello,
Recently I am running my programs in QEMU (x86_64) with “-accel=kvm”. The QEMU
version is 6.0.0.
I run my programs in two ways:
1. I pass through my device through vfio-pci to QEMU, this way works well.
2. I write an emulated PCI device for QEMU, and run my programs on the
emulated PCI device.
This crashes when the code try to do memory copy to PCI device when the
data length is longer than 16 bytes.
While the passthrough device works well for the same situation.
After dump the assembly code. I noticed when the data is <= 16 bytes, the
mov assembly code is chosen, and it works well.
When the data is > 16 bytes, the vmovdqu assembly code is chosen, and it
crashes with “illegal operand”.
Given the code and data are exactly same for both passthrough device and
emulated device. I am curious about why this happens.
After turn on kernel trace for kvm by echo kvm:*
/sys/kernel/debug/tracing/set_event
And rerun the QEMU and my code for both passthrough device and emulated device,
I noticed that:
1) for passthrough device, I didn’t see any trace events related to my gva
and gpa. This makes me think that the memory copy to PCI device went through
different code path . It is handled by the guest OS without exit to VMX.
2) for emulated device, if I use compiler flag target-feature=-avx,-avx2 to
force compiler use mov assembly code, I can see the memory copy goes through
the KVM_EXIT_MMIO, and everything works well.
if I don’t force the compiler use mov , the compiler just chooses the
vmovdqu , which just crash the programs, and no KVM_EXIT_MMIO related to my
memory copy appears in the trace events. Looks like the guest OS handles the
crash.
Any clue about why the vmovdqu works for passthrough device but not work for
emulated device.
Thanks
