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

[Sean Christopherson <seanjc@google.com>]

On Thu, Aug 11, 2022, Adam Dunlap 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.
> 
> Signed-off-by: Adam Dunlap <acdunlap@google.com>
> Reviewed-by: Marc Orr <marcorr@google.com>
> Reviewed-by: Jacob Xu <jacobhxu@google.com>
> ---
>  arch/x86/include/asm/apic.h | 13 ++++++++++++-
>  1 file changed, 12 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/include/asm/apic.h b/arch/x86/include/asm/apic.h
> index 3415321c8240..281db79e76a9 100644
> --- a/arch/x86/include/asm/apic.h
> +++ b/arch/x86/include/asm/apic.h
> @@ -109,7 +109,18 @@ static inline void native_apic_mem_write(u32 reg, u32 v)
>  
>  static inline u32 native_apic_mem_read(u32 reg)
>  {
> -	return *((volatile u32 *)(APIC_BASE + reg));
> +	volatile u32 *addr = (volatile u32 *)(APIC_BASE + reg);
> +	u32 out;
> +
> +	/*
> +	 * Functionally, what we want to do is simply return *addr. However,
> +	 * this accesses an MMIO which may need to be emulated in some cases.
> +	 * The emulator doesn't necessarily support all instructions, so we
> +	 * force the read from addr to use a mov instruction.
> +	 */
> +	asm_inline("movl %1, %0" : "=r"(out) : "m"(*addr));
> +
> +	return out;

Can't this just be:

	return readl((void __iomem *)(APIC_BASE + reg));