Re: [PATCH v2 1/3] KVM: x86: Deflect unknown MSR accesses to user space

[Alexander Graf <graf@amazon.com>]

On 31.07.20 00:42, Jim Mattson wrote:
> 
> On Wed, Jul 29, 2020 at 4:59 PM Alexander Graf <graf@amazon.com> wrote:
>>
>> MSRs are weird. Some of them are normal control registers, such as EFER.
>> Some however are registers that really are model specific, not very
>> interesting to virtualization workloads, and not performance critical.
>> Others again are really just windows into package configuration.
>>
>> Out of these MSRs, only the first category is necessary to implement in
>> kernel space. Rarely accessed MSRs, MSRs that should be fine tunes against
>> certain CPU models and MSRs that contain information on the package level
>> are much better suited for user space to process. However, over time we have
>> accumulated a lot of MSRs that are not the first category, but still handled
>> by in-kernel KVM code.
>>
>> This patch adds a generic interface to handle WRMSR and RDMSR from user
>> space. With this, any future MSR that is part of the latter categories can
>> be handled in user space.
>>
>> Furthermore, it allows us to replace the existing "ignore_msrs" logic with
>> something that applies per-VM rather than on the full system. That way you
>> can run productive VMs in parallel to experimental ones where you don't care
>> about proper MSR handling.
>>
>> Signed-off-by: Alexander Graf <graf@amazon.com>
> 
> Can we just drop em_wrmsr and em_rdmsr? The in-kernel emulator is
> already incomplete, and I don't think there is ever a good reason for
> kvm to emulate RDMSR or WRMSR if the VM-exit was for some other reason
> (and we shouldn't end up here if the VM-exit was for RDMSR or WRMSR).
> Am I missing something?

On certain combinations of CPUs and guest modes, such as real mode on 
pre-Nehalem(?) at least, we are running all guest code through the 
emulator and thus may encounter a RDMSR or WRMSR instruction. I *think* 
we also do so for big real mode on more modern CPUs, but I'm not 100% sure.

> You seem to be assuming that the instruction at CS:IP will still be
> RDMSR (or WRMSR) after returning from userspace, and we will come
> through kvm_{get,set}_msr_user_space again at the next KVM_RUN. That
> isn't necessarily the case, for a variety of reasons. I think the

Do you have a particular situation in mind where that would not be the 
case and where we would still want to actually complete an MSR operation 
after the environment changed?

> 'completion' of the userspace instruction emulation should be done
> with the complete_userspace_io [sic] mechanism instead.

Hm, that would avoid a roundtrip into guest mode, but add a cycle 
through the in-kernel emulator. I'm not sure that's a net win quite yet.

> 
> I'd really like to see this mechanism apply only in the case of
> invalid/unknown MSRs, and not for illegal reads/writes as well.

Why? Any #GP inducing MSR access will be on the slow path. What's the 
problem if you get a few more of them in user space that you just bounce 
back as failing, so they actually do inject a fault?

Alex



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