linux-kernel - Re: [PATCH v7 08/12] KVM/x86/vPMU: Add APIs to support host save/restore the guest lbr stack

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <5D259F92.6030107@intel.com>
Date:   Wed, 10 Jul 2019 16:19:30 +0800
From:   Wei Wang <wei.w.wang@...el.com>
To:     Peter Zijlstra <peterz@...radead.org>
CC:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        pbonzini@...hat.com, ak@...ux.intel.com, kan.liang@...el.com,
        mingo@...hat.com, rkrcmar@...hat.com, like.xu@...el.com,
        jannh@...gle.com, arei.gonglei@...wei.com, jmattson@...gle.com
Subject: Re: [PATCH v7 08/12] KVM/x86/vPMU: Add APIs to support host save/restore
 the guest lbr stack

On 07/09/2019 08:19 PM, Peter Zijlstra wrote:
>
>> For the lbr feature, could we thought of it as first come, first served?
>> For example, if we have 2 host threads who want to use lbr at the same time,
>> I think one of them would simply fail to use.
>>
>> So if guest first gets the lbr, host wouldn't take over unless some
>> userspace command (we added to QEMU) is executed to have the vCPU
>> actively stop using lbr.
> Doesn't work that way.
>
> Say you start KVM with LBR emulation, it creates this task event, it
> gets the LBR (nobody else wants it) and the guest works and starts using
> the LBR.
>
> Then the host creates a CPU LBR event and the vCPU suddenly gets denied
> the LBR and the guest no longer functions correctly.
>
> Or you should fail to VMENTER, in which case you starve the guest, but
> at least it doesn't malfunction.
>

Ok, I think we could go with the first option for now, like this:

When there are other host threads starting to use lbr, host perf notifies
kvm to disable the guest use of lbr (add a notifier_block), which includes:
  - cancel passing through lbr msrs to guest
  - upon guest access to the lbr msr (will trap to KVM now), return #GP
    (or 0).


Best,
Wei