Re: [PATCH 6/8] x86/pv: allow reading FEATURE_CONTROL MSR

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 17/08/2020 16:57, Roger Pau Monne wrote:
> Linux PV guests will attempt to read the FEATURE_CONTROL MSR, report
> no features enabled or available, and that the MSR is already locked.
>
> Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
> ---
>  xen/arch/x86/pv/emul-priv-op.c | 4 ++++
>  1 file changed, 4 insertions(+)
>
> diff --git a/xen/arch/x86/pv/emul-priv-op.c b/xen/arch/x86/pv/emul-priv-op.c
> index 554a95ae8d..76c878b677 100644
> --- a/xen/arch/x86/pv/emul-priv-op.c
> +++ b/xen/arch/x86/pv/emul-priv-op.c
> @@ -879,6 +879,10 @@ static int read_msr(unsigned int reg, uint64_t *val,
>              *val |= APIC_BASE_BSP;
>          return X86EMUL_OKAY;
>  
> +    case MSR_IA32_FEATURE_CONTROL:
> +        *val = IA32_FEATURE_CONTROL_LOCK;
> +        return X86EMUL_OKAY;

This isn't quite right.  This is an Intel-like MSR only, and should #GP
for AMD/Hygon.

It would be better to move it to the common guest_rdmsr() function, as
the two helpers (vmce_lmce, and nested_virt) used to construct it in the
vmx code are already generic.

This also helps progress the work to drop all the legacy MSR handling.

~Andrew