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

[Dave Hansen <dave.hansen@intel.com>]

On 9/14/22 04:13, Peter Gonda wrote:
> On Thu, Sep 8, 2022 at 6:05 PM Adam Dunlap <acdunlap@google.com> 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".

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.