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

[Marc Orr]

On Wed, Oct 5, 2022 at 6:29 AM Tom Lendacky <thomas.lendacky@xxxxxxx> wrote:
>
> On 10/3/22 18:11, H. Peter Anvin wrote:
> > On October 3, 2022 4:01:01 PM PDT, Adam Dunlap <acdunlap@xxxxxxxxxx> wrote:
> >> Thanks for all the responses. Is the consensus that we should use the
> >> readl function here or instead use inline assembly directly as in the patch
> >> I originally sent out:
> >>
> >> asm_inline("movl %1, %0" : "=r"(out) : "m"(*addr));
> >>
> >> ? The readl function has this exact same code, I'm just not sure
> >> which version fits better stylistically.
> >
> > Is mov with an arbitrary addressing mode still acceptable for whatever is causing this problem?
>
> The acceptable forms of MOV are covered by insn_decode_mmio() in
> arch/x86/lib/insn-eval.c.

Is this blocked on an item? There seems to be consensus that this
patch fixes a bug and is taking the right high-level approach (i.e.,
change the guest code to avoid triggering a sequence that isn't
supported under CVM exception-based emulation). Without something like
this, we weren't able to build the kernel w/ CLANG when it is
configured to run under SEV-ES.

We sent out two versions of the patch. One that does the mov directly
[1] and a second that calls readl [2]. Is one of these two patches
acceptable? Or do we need to follow up on something?

[1] https://lore.kernel.org/lkml/0D6A1E49-F21B-42AA-BBBF-13BFC308BB1E@xxxxxxxxx/T/
[2] https://lore.kernel.org/all/20220812183501.3555820-1-acdunlap@xxxxxxxxxx/

Thanks,
Marc