Re: [Xen-devel] [PATCH v3 5/9] x86/vmx: Improvements to LBR MSR handling

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 27.06.18 at 10:43, <andrew.cooper3@xxxxxxxxxx> wrote:
> The main purpose of this patch is to only ever insert the LBR MSRs into the
> guest load/save list once, as a future patch wants to change the behaviour of
> vmx_add_guest_msr().
> 
> The repeated processing of lbr_info and the guests MSR load/save list is
> redundant, and a guest using LBR itself will have to re-enable
> MSR_DEBUGCTL.LBR in its #DB handler, meaning that Xen will repeat this
> redundant processing every time the guest gets a debug exception.
> 
> Rename lbr_fixup_enabled to lbr_flags to be a little more generic, and use one
> bit to indicate that the MSRs have been inserted into the load/save list.
> Shorten the existing FIXUP* identifiers to reduce code volume.
> 
> Furthermore, handing the guest #MC on an error isn't a legitimate action.  Two
> of the three failure cases are definitely hypervisor bugs, and the third is a
> boundary case which shouldn't occur in practice.  The guest also won't execute
> correctly, so handle errors by cleanly crashing the guest.
> 
> Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>

Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
with one minor remark:

> @@ -3097,30 +3098,64 @@ static int vmx_msr_write_intercept(unsigned int msr, 
> uint64_t msr_content)
>              if ( vpmu_do_wrmsr(msr, msr_content, supported) )
>                  break;
>          }
> -        if ( msr_content & IA32_DEBUGCTLMSR_LBR )
> +
> +        /*
> +         * When a guest first enables LBR, arrange to save and restore the 
> LBR
> +         * MSRs and allow the guest direct access.
> +         *
> +         * MSR_DEBUGCTL and LBR has existed almost long as MSRs have existed,

... as long as ...

Jan



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