Re: [PATCH v2 RESEND] x86/asm: Force native_apic_mem_read to use mov

[Peter Gonda]

On Wed, Sep 14, 2022 at 5:22 PM Sean Christopherson <seanjc@xxxxxxxxxx> wrote:
>
> On Wed, Sep 14, 2022, Dave Hansen wrote:
> > On 9/14/22 04:13, Peter Gonda wrote:
> > > On Thu, Sep 8, 2022 at 6:05 PM Adam Dunlap <acdunlap@xxxxxxxxxx> wrote:
> > >> Previously, when compiled with clang, native_apic_mem_read gets inlined
> > >> into __xapic_wait_icr_idle and optimized to a testl instruction. When
> > >> run in a VM with SEV-ES enabled, it attempts to emulate this
> > >> instruction, but the emulator does not support it. Instead, use inline
> > >> assembly to force native_apic_mem_read to use the mov instruction which
> > >> is supported by the emulator.
> > > This seems to be an issue with the SEV-ES in guest #VC handler's
> > > "emulator" right?
> >
> > No.
> >
> > It's not just an SEV-ES thing.  It's a problem for TDX and _probably_ a
> > problem for normal virtualization where it's a host-side issue.  Kirill
> > wrote a lot of great background information in here:
> >
> > > https://lore.kernel.org/all/164946765464.4207.3715751176055921036.tip-bot2@tip-bot2/
> >
> > So, the question is not "should we extend the MMIO instruction decoders
> > to handle one more instruction?".  It is "should we extend the MMIO
> > decoders to handle *ALL* memory read instructions?"
> >
> > That's an even more emphatic "NO".
>
> +1, keep the guest-side decoding as simple as possible.
>
> > readl() seems to be the right thing to do.  Also, Dear TDX, SEV and virt
> > folks: please look for more of these.  They're going to bite you sooner
> > or later.  You should have caught this one before now.

Thanks for the context here Dave. Fixing the offending MMIO
instruction here makes sense.