Re: [PATCH] KVM: x86: work around QEMU issue with synthetic CPUID leaves

[Maxim Levitsky]

On Fri, 2022-04-29 at 15:25 -0400, Paolo Bonzini wrote:
> Synthesizing AMD leaves up to 0x80000021 caused problems with QEMU,
> which assumes the *host* CPUID[0x80000000].EAX is higher or equal
> to what KVM_GET_SUPPORTED_CPUID reports.
> 
> This causes QEMU to issue bogus host CPUIDs when preparing the input
> to KVM_SET_CPUID2.  It can even get into an infinite loop, which is
> only terminated by an abort():
> 
>    cpuid_data is full, no space for cpuid(eax:0x8000001d,ecx:0x3e)
> 
> To work around this, only synthesize those leaves if 0x8000001d exists
> on the host.  The synthetic 0x80000021 leaf is mostly useful on Zen2,
> which satisfies the condition.
> 
> Fixes: f144c49e8c39 ("KVM: x86: synthesize CPUID leaf 0x80000021h if useful")
> Reported-by: Maxim Levitsky <mlevitsk@xxxxxxxxxx>
> Signed-off-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>
> ---
>  arch/x86/kvm/cpuid.c | 19 ++++++++++++++-----
>  1 file changed, 14 insertions(+), 5 deletions(-)
> 
> diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
> index b24ca7f4ed7c..598334ed5fbc 100644
> --- a/arch/x86/kvm/cpuid.c
> +++ b/arch/x86/kvm/cpuid.c
> @@ -1085,12 +1085,21 @@ static inline int __do_cpuid_func(struct kvm_cpuid_array *array, u32 function)
>  	case 0x80000000:
>  		entry->eax = min(entry->eax, 0x80000021);
>  		/*
> -		 * Serializing LFENCE is reported in a multitude of ways,
> -		 * and NullSegClearsBase is not reported in CPUID on Zen2;
> -		 * help userspace by providing the CPUID leaf ourselves.
> +		 * Serializing LFENCE is reported in a multitude of ways, and
> +		 * NullSegClearsBase is not reported in CPUID on Zen2; help
> +		 * userspace by providing the CPUID leaf ourselves.
> +		 *
> +		 * However, only do it if the host has CPUID leaf 0x8000001d.
> +		 * QEMU thinks that it can query the host blindly for that
> +		 * CPUID leaf if KVM reports that it supports 0x8000001d or
> +		 * above.  The processor merrily returns values from the
> +		 * highest Intel leaf which QEMU tries to use as the guest's
> +		 * 0x8000001d.  Even worse, this can result in an infinite
> +		 * loop if said highest leaf has no subleaves indexed by ECX.

Very small nitpick: It might be useful to add a note that qemu does this only for the
leaf 0x8000001d.

>  		 */
> -		if (static_cpu_has(X86_FEATURE_LFENCE_RDTSC)
> -		    || !static_cpu_has_bug(X86_BUG_NULL_SEG))
> +		if (entry->eax >= 0x8000001d &&
> +		    (static_cpu_has(X86_FEATURE_LFENCE_RDTSC)
> +		     || !static_cpu_has_bug(X86_BUG_NULL_SEG)))
>  			entry->eax = max(entry->eax, 0x80000021);
>  		break;
>  	case 0x80000001:

Reviewed-by: Maxim Levitsky <mlevitsk@xxxxxxxxxx>

Best regards,
	Maxim Levitsky