On Wed, May 8, 2019 at 9:08 AM Sean Christopherson
<sean.j.christopherson@xxxxxxxxx> wrote:
>
> The RDPMC-exiting control is dependent on the existence of the RDPMC
> instruction itself, i.e. is not tied to the "Architectural Performance
> Monitoring" feature. For all intents and purposes, the control exists
> on all CPUs with VMX support since RDPMC also exists on all VCPUs with
> VMX supported. Per Intel's SDM:
>
> The RDPMC instruction was introduced into the IA-32 Architecture in
> the Pentium Pro processor and the Pentium processor with MMX technology.
> The earlier Pentium processors have performance-monitoring counters, but
> they must be read with the RDMSR instruction.
>
> Because RDPMC-exiting always exists, KVM requires the control and refuses
> to load if it's not available. As a result, hiding the PMU from a guest
> breaks nested virtualization if the guest attemts to use KVM.
Is it true that the existence of instruction <X> implies the
availaibility of the VM-execution control <X>-exiting (if such a
VM-execution control exists)? What about WBINVD? That instruction has
certainly been around forever, but there were VMX-capable processors
that did not support WBINVD-exiting.
Having said that, I think our hands are tied by the assumptions made
by existing hypervisors, whether or not those assumptions are true.
(VMware's VMM, for instance, requires MONITOR-exiting and
MWAIT-exiting even when MONITOR/MWAIT are not enumerated by CPUID.)
> While it's not explicitly stated in the RDPMC pseudocode, the VM-Exit
> check for RDPMC-exiting follows standard fault vs. VM-Exit prioritization
> for privileged instructions, e.g. occurs after the CPL/CR0.PE/CR4.PCE
> checks, but before the counter referenced in ECX is checked for validity.
>
> In other words, the original KVM behavior of injecting a #GP was correct,
> and the KVM unit test needs to be adjusted accordingly, e.g. eat the #GP
> when the unit test guest (L3 in this case) executes RDPMC without
> RDPMC-exiting set in the unit test host (L2).
>
> This reverts commit e51bfdb68725dc052d16241ace40ea3140f938aa.
>
> Fixes: e51bfdb68725 ("KVM: nVMX: Expose RDPMC-exiting only when guest supports PMU")
> Reported-by: David Hill <hilld@xxxxxxxxxxxxxxx>
> Cc: Saar Amar <saaramar@xxxxxxxxxxxxx>
> Cc: Mihai Carabas <mihai.carabas@xxxxxxxxxx>
> Cc: Jim Mattson <jmattson@xxxxxxxxxx>
> Cc: Liran Alon <liran.alon@xxxxxxxxxx>
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Sean Christopherson <sean.j.christopherson@xxxxxxxxx>
Reviewed-by: Jim Mattson <jmattson@xxxxxxxxxx>
