Re: [PATCH] x86/hvm/viridian: fix the TLB flush hypercall

[Andrew Cooper <andrew.cooper3@citrix.com>]

On 16/03/16 13:00, Paul Durrant wrote:
> Commit b38d426a "flush remote tlbs by hypercall" add support to allow
> Windows to request flush of remote TLB via hypercall rather than IPI.
> Unfortunately it seems that this code was broken in a couple of ways:
>
> 1) The allocation of the per-vcpu flush mask is gated on whether the
>    domain has viridian features enabled but the call to allocate is
>    made before the toolstack has enabled those features. This results
>    in a NULL pointer dereference.
>
> 2) One of the flush hypercall variants is a rep op, but the code
>    does not update the output data with the reps completed. Hence the
>    guest will spin repeatedly making the hypercall because it believes
>    it has uncompleted reps.
>
> This patch fixes both of these issues and also adds a check to make
> sure the current vCPU is not included in the flush mask (since there's
> clearly no need for the CPU to IPI itself).

Thinking more about this, the asid flush does serve properly for TLB
flushing.  Why do we then subsequently use flush_tlb_mask(), as opposed
to a less heavyweight alternative like smp_send_event_check_mask() ?

~Andrew

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