Re: [PATCH] x86/domctl: fix maximum number of MSRs in XEN_DOMCTL_{get,set}_vcpu_msrs

[Jan Beulich <jbeulich@xxxxxxxx>]

On 08.10.2024 10:37, Roger Pau Monne wrote:
> Since the addition of the MSR_AMD64_DR{1-4}_ADDRESS_MASK MSRs to the
> msrs_to_send array, the calculations for the maximum number of MSRs that
> the hypercall can handle is off by 4.
> 
> Remove the addition of 4 to the maximum number of MSRs that
> XEN_DOMCTL_{set,get}_vcpu_msrs supports, as those are already part of the
> array.
> 
> A further adjustment could be to subtract 4 from the maximum size if the DBEXT
> CPUID feature is not exposed to the guest, but guest_{rd,wr}msr() will already
> perform that check when fetching or loading the MSRs.  The maximum array is
> used to indicate the caller of the buffer it needs to allocate in the get 
> case,
> and as an early input sanitation in the set case, using a buffer size slightly
> lager than required is not an issue.
> 
> Fixes: 86d47adcd3c4 ('x86/msr: Handle MSR_AMD64_DR{0-3}_ADDRESS_MASK in the 
> new MSR infrastructure')
> Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>

Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>

> ---
> I'm tempted to just get rid of nr_msrs and use ARRAY_SIZE(msrs_to_send)
> instead, but refrained from doing it.

I think the variable would indeed better go away now. Yet that doesn't
necessarily need to happen right here.

Jan