Re: [PATCH v2 1/3] xen/pv: allow pmu msr accesses to cause GP

[Jan Beulich <jbeulich@xxxxxxxx>]

On 04.10.2022 10:43, Juergen Gross wrote:
> Today pmu_msr_read() and pmu_msr_write() fall back to the safe variants
> of read/write MSR in case the MSR access isn't emulated via Xen. Allow
> the caller to select the potentially faulting variant by passing NULL
> for the error pointer.
> 
> Restructure the code to make it more readable.
> 
> Signed-off-by: Juergen Gross <jgross@xxxxxxxx>

I think the title (and to some degree also the description) is misleading:
The property we care about here isn't whether an MSR access would raise
#GP (we can't control that), but whether that #GP would be recovered from.

> --- a/arch/x86/xen/pmu.c
> +++ b/arch/x86/xen/pmu.c
> @@ -131,6 +131,9 @@ static inline uint32_t get_fam15h_addr(u32 addr)
>  
>  static inline bool is_amd_pmu_msr(unsigned int msr)
>  {
> +     if (boot_cpu_data.x86_vendor == X86_VENDOR_INTEL)
> +             return false;

I understand this and ...

> @@ -144,6 +147,9 @@ static int is_intel_pmu_msr(u32 msr_index, int *type, int 
> *index)
>  {
>       u32 msr_index_pmc;
>  
> +     if (boot_cpu_data.x86_vendor != X86_VENDOR_INTEL)
> +             return false;

... this matches prior behavior, but may I suggest that while moving
these here you at least accompany them by a comment clarifying that
these aren't really correct? We'd come closer if is_amd_pmu_msr()
accepted AMD and Hygon, while is_intel_pmu_msr() may want to accept
Intel and Centaur (but I understand this would be largely orthogonal,
hence the suggestion towards comments). In the hypervisor we kind of
also support Shanghai, but I wonder whether we wouldn't better rip
out that code as unmaintained.

Jan