linux-kernel - Re: [PATCH v2 1/4] KVM: nVMX: Properly expose ENABLE_USR_WAIT

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Message-ID: <CALMp9eSsH5UO4=NXOwEWzyqswBQacX7K13X+Tzy6bTQDBatc-A@mail.gmail.com>
Date:   Tue, 13 Dec 2022 10:08:33 -0800
From:   Jim Mattson <jmattson@...gle.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, Aaron Lewis <aaronlewis@...gle.com>,
        Yu Zhang <yu.c.zhang@...ux.intel.com>
Subject: Re: [PATCH v2 1/4] KVM: nVMX: Properly expose ENABLE_USR_WAIT_PAUSE
 control to L1

On Mon, Dec 12, 2022 at 10:23 PM Sean Christopherson <seanjc@...gle.com> wrote:
>
> Set ENABLE_USR_WAIT_PAUSE in KVM's supported VMX MSR configuration if the
> feature is supported in hardware and enabled in KVM's base, non-nested
> configuration, i.e. expose ENABLE_USR_WAIT_PAUSE to L1 if it's supported.
> This fixes a bug where saving/restoring, i.e. migrating, a vCPU will fail
> if WAITPKG (the associated CPUID feature) is enabled for the vCPU, and
> obviously allows L1 to enable the feature for L2.
>
> KVM already effectively exposes ENABLE_USR_WAIT_PAUSE to L1 by stuffing
> the allowed-1 control ina vCPU's virtual MSR_IA32_VMX_PROCBASED_CTLS2 when
> updating secondary controls in response to KVM_SET_CPUID(2), but (a) that
> depends on flawed code (KVM shouldn't touch VMX MSRs in response to CPUID
> updates) and (b) runs afoul of vmx_restore_control_msr()'s restriction
> that the guest value must be a strict subset of the supported host value.
>
> Although no past commit explicitly enabled nested support for WAITPKG,
> doing so is safe and functionally correct from an architectural
> perspective as no additional KVM support is needed to virtualize TPAUSE,
> UMONITOR, and UMWAIT for L2 relative to L1, and KVM already forwards
> VM-Exits to L1 as necessary (commit bf653b78f960, "KVM: vmx: Introduce
> handle_unexpected_vmexit and handle WAITPKG vmexit").
>
> Note, KVM always keeps the hosts MSR_IA32_UMWAIT_CONTROL resident in
> hardware, i.e. always runs both L1 and L2 with the host's power management
> settings for TPAUSE and UMWAIT.  See commit bf09fb6cba4f ("KVM: VMX: Stop
> context switching MSR_IA32_UMWAIT_CONTROL") for more details.
>
> Fixes: e69e72faa3a0 ("KVM: x86: Add support for user wait instructions")
> Cc: stable@...r.kernel.org
> Reported-by: Aaron Lewis <aaronlewis@...gle.com>
> Reported-by: Yu Zhang <yu.c.zhang@...ux.intel.com>
> Signed-off-by: Sean Christopherson <seanjc@...gle.com>
Reviewed-by: Jim Mattson <jmattson@...gle.com>