Re: [PATCH v3 07/12] KVM: x86: Ensure the MSR bitmap never clears userspace tracked MSRs

[Alexander Graf <graf@amazon.com>]

On 18.08.20 23:15, Aaron Lewis wrote:
> 
> SDM volume 3: 24.6.9 "MSR-Bitmap Address" and APM volume 2: 15.11 "MS
> intercepts" describe MSR permission bitmaps.  Permission bitmaps are
> used to control whether an execution of rdmsr or wrmsr will cause a
> vm exit.  For userspace tracked MSRs it is required they cause a vm
> exit, so the host is able to forward the MSR to userspace.  This change
> adds vmx/svm support to ensure the permission bitmap is properly set to
> cause a vm_exit to the host when rdmsr or wrmsr is used by one of the
> userspace tracked MSRs.  Also, to avoid repeatedly setting them,
> kvm_make_request() is used to coalesce these into a single call.
> 
> Signed-off-by: Aaron Lewis <aaronlewis@google.com>
> Reviewed-by: Oliver Upton <oupton@google.com>

This is incomplete, as it doesn't cover all of the x2apic registers. 
There are also a few MSRs that IIRC are handled differently from this 
logic, such as EFER.

I'm really curious if this is worth the effort? I would be inclined to 
say that MSRs that KVM has direct access for need special handling one 
way or another.


Alex



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