Re: [Question] int3 instruction generates a #UD in SEV VM

[Tom Lendacky]

On 7/31/23 09:30, Sean Christopherson wrote:
> On Sat, Jul 29, 2023, wuzongyong wrote:
> > Hi,
> > I am writing a firmware in Rust to support SEV based on project td-shim[1].
> > But when I create a SEV VM (just SEV, no SEV-ES and no SEV-SNP) with the firmware,
> > the linux kernel crashed because the int3 instruction in int3_selftest() cause a
> > #UD.
>
> ...
>
> > BTW, if a create a normal VM without SEV by qemu & OVMF, the int3 instruction always generates a
> > #BP.
> > So I am confused now about the behaviour of int3 instruction, could anyone help to explain the behaviour?
> > Any suggestion is appreciated!
>
> Have you tried my suggestions from the other thread[*]?
>
>    : > > I'm curious how this happend. I cannot find any condition that would
>    : > > cause the int3 instruction generate a #UD according to the AMD's spec.
>    :
>    : One possibility is that the value from memory that gets executed diverges from the
>    : value that is read out be the #UD handler, e.g. due to patching (doesn't seem to
>    : be the case in this test), stale cache/tlb entries, etc.
>    :
>    : > > BTW, it worked nomarlly with qemu and ovmf.
>    : >
>    : > Does this happen every time you boot the guest with your firmware? What
>    : > processor are you running on?
>    :
>    : And have you ruled out KVM as the culprit?  I.e. verified that KVM is NOT injecting
>    : a #UD.  That obviously shouldn't happen, but it should be easy to check via KVM
>    : tracepoints.

I have a feeling that KVM is injecting the #UD, but it will take 
instrumenting KVM to see which path the #UD is being injected from.

Wu Zongyo, can you add some instrumentation to figure that out if the 
trace points towards KVM injecting the #UD?

Thanks,
Tom

> [*] https://lore.kernel.org/all/ZMFd5kkehlkIfnBA@xxxxxxxxxx