linux-kernel - Re: [PATCH] KVM: x86: Deflect unknown MSR accesses to user space

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Message-ID: <87pn8ellp6.fsf@vitty.brq.redhat.com>
Date:   Wed, 29 Jul 2020 11:22:45 +0200
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     Alexander Graf <graf@...zon.com>, Jim Mattson <jmattson@...gle.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Jonathan Corbet <corbet@....net>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Joerg Roedel <joro@...tes.org>, kvm list <kvm@...r.kernel.org>,
        linux-doc@...r.kernel.org, LKML <linux-kernel@...r.kernel.org>,
        Aaron Lewis <aaronlewis@...gle.com>
Subject: Re: [PATCH] KVM: x86: Deflect unknown MSR accesses to user space

Alexander Graf <graf@...zon.com> writes:

> On 29.07.20 10:23, Vitaly Kuznetsov wrote:
>> 
>> 
>> 
>> Jim Mattson <jmattson@...gle.com> writes:
>> 
>>> On Tue, Jul 28, 2020 at 5:41 AM Alexander Graf <graf@...zon.com> wrote:
>>>>
>> 
>> ...
>> 
>>>> While it does feel a bit overengineered, it would solve the problem that
>>>> we're turning in-KVM handled MSRs into an ABI.
>>>
>>> It seems unlikely that userspace is going to know what to do with a
>>> large number of MSRs. I suspect that a small enumerated list will
>>> suffice.
>> 
>> The list can also be 'wildcarded', i.e.
>> {
>>   u32 index;
>>   u32 mask;
>>   ...
>> }
>> 
>> to make it really short.
>
> I like the idea of wildcards, but I can't quite wrap my head around how 
> we would implement ignore_msrs in user space with them?
>

For that I think we can still deflect all unknown MSR accesses to
userspace (when the CAP is enabled of course ) but MSRs which are on the
list will *have to be deflected*, i.e. KVM can't handle them internally
without consulting with userspace.

We can make it tunable through a parameter for CAP enablement if needed.

-- 
Vitaly