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From: Alexander Graf <graf@amazon.com>
To: Aaron Lewis <aaronlewis@google.com>
Cc: Jim Mattson <jmattson@google.com>, kvm list <kvm@vger.kernel.org>
Subject: Re: [PATCH v3 07/12] KVM: x86: Ensure the MSR bitmap never clears userspace tracked MSRs
Date: Fri, 21 Aug 2020 00:04:09 +0200	[thread overview]
Message-ID: <d75a3862-d4f4-e057-5d45-9edcb3f9b696@amazon.com> (raw)
In-Reply-To: <CAAAPnDFEKOQjTKcmkFjP6hr6dgmR-61NL_W9=7Fs0THdOOJ7+Q@mail.gmail.com>



On 20.08.20 02:18, Aaron Lewis wrote:
> 
> On Wed, Aug 19, 2020 at 8:26 AM Alexander Graf <graf@amazon.com> wrote:
>>
>>
>>
>> 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.
>>
> 
> Can you please elaborate on this?  It was my understanding that the
> permission bitmap covers the x2apic registers.  Also, I’m not sure how

So x2apic MSR passthrough is configured specially:

 
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/arch/x86/kvm/vmx/vmx.c#n3796

and I think not handled by this patch?

> EFER is handled differently, but I see there is a separate
> conversation on that.

EFER is a really special beast in VT.

> This effort does seem worthwhile as it ensures userspace is able to
> manage the MSRs it is requesting, and will remain that way in the
> future.

I couldn't see why any of the passthrough MSRs are relevant to user 
space, but I tend to agree that it makes everything more consistent.


Alex



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  reply	other threads:[~2020-08-20 22:04 UTC|newest]

Thread overview: 49+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-08-18 21:15 [PATCH v3 00/12] Allow userspace to manage MSRs Aaron Lewis
2020-08-18 21:15 ` [PATCH v3 01/12] KVM: x86: Deflect unknown MSR accesses to user space Aaron Lewis
2020-08-19  8:42   ` Alexander Graf
2020-08-18 21:15 ` [PATCH v3 02/12] KVM: x86: Introduce allow list for MSR emulation Aaron Lewis
2020-08-19  8:53   ` Alexander Graf
2020-08-31 10:39   ` Dan Carpenter
2020-08-31 10:39     ` Dan Carpenter
2020-08-31 10:39     ` Dan Carpenter
2020-09-01 19:13     ` Alexander Graf
2020-09-02  7:31       ` Dan Carpenter
2020-09-02  7:31         ` Dan Carpenter
2020-09-02  7:31         ` Dan Carpenter
2020-08-18 21:15 ` [PATCH v3 03/12] KVM: selftests: Add test for user space MSR handling Aaron Lewis
2020-08-18 21:15 ` [PATCH v3 04/12] KVM: x86: Add ioctl for accepting a userspace provided MSR list Aaron Lewis
2020-08-19  9:00   ` Alexander Graf
2020-08-20 17:30     ` Jim Mattson
2020-08-20 21:49       ` Alexander Graf
2020-08-20 22:28         ` Jim Mattson
2020-08-18 21:15 ` [PATCH v3 05/12] KVM: x86: Add support for exiting to userspace on rdmsr or wrmsr Aaron Lewis
2020-08-19 10:25   ` Alexander Graf
2020-08-20 18:17   ` Jim Mattson
2020-08-20 21:59     ` Alexander Graf
2020-08-20 22:55       ` Jim Mattson
2020-08-21 17:58         ` Jim Mattson
2020-08-24  1:35           ` Alexander Graf
2020-08-24 17:23             ` Jim Mattson
2020-08-24 18:09               ` Alexander Graf
2020-08-24 18:34                 ` Jim Mattson
2020-08-18 21:15 ` [PATCH v3 06/12] KVM: x86: Prepare MSR bitmaps for userspace tracked MSRs Aaron Lewis
2020-08-18 21:15 ` [PATCH v3 07/12] KVM: x86: Ensure the MSR bitmap never clears " Aaron Lewis
2020-08-19  1:12   ` kernel test robot
2020-08-19  1:12     ` kernel test robot
2020-08-19  1:12   ` [RFC PATCH] KVM: x86: vmx_set_user_msr_intercept() can be static kernel test robot
2020-08-19  1:12     ` kernel test robot
2020-08-19 15:26   ` [PATCH v3 07/12] KVM: x86: Ensure the MSR bitmap never clears userspace tracked MSRs Alexander Graf
2020-08-20  0:18     ` Aaron Lewis
2020-08-20 22:04       ` Alexander Graf [this message]
2020-08-20 22:35         ` Jim Mattson
2020-08-21 14:27           ` Aaron Lewis
2020-08-21 16:07             ` Alexander Graf
2020-08-21 16:43               ` Aaron Lewis
2020-08-26 15:48   ` kernel test robot
2020-08-26 15:48     ` kernel test robot
2020-08-18 21:15 ` [PATCH v3 08/12] selftests: kvm: Fix the segment descriptor layout to match the actual layout Aaron Lewis
2020-08-18 21:15 ` [PATCH v3 09/12] selftests: kvm: Clear uc so UCALL_NONE is being properly reported Aaron Lewis
2020-08-19  9:13   ` Andrew Jones
2020-08-18 21:15 ` [PATCH v3 10/12] selftests: kvm: Add exception handling to selftests Aaron Lewis
2020-08-18 21:15 ` [PATCH v3 11/12] selftests: kvm: Add a test to exercise the userspace MSR list Aaron Lewis
2020-08-18 21:15 ` [PATCH v3 12/12] selftests: kvm: Add emulated rdmsr, wrmsr tests Aaron Lewis

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