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From: Paolo Bonzini <pbonzini@redhat.com>
To: Sean Christopherson <seanjc@google.com>
Cc: Vitaly Kuznetsov <vkuznets@redhat.com>,
	Wanpeng Li <wanpengli@tencent.com>,
	Jim Mattson <jmattson@google.com>, Joerg Roedel <joro@8bytes.org>,
	kvm@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH] KVM: x86: Allow guest to set EFER.NX=1 on non-PAE 32-bit kernels
Date: Fri, 6 Aug 2021 13:58:00 +0200	[thread overview]
Message-ID: <ec7165f9-a008-e11d-a7d4-c12503b08434@redhat.com> (raw)
In-Reply-To: <20210805183804.1221554-1-seanjc@google.com>

On 05/08/21 20:38, Sean Christopherson wrote:
> Fast forward to today, and KVM has long since stopped running the guest
> with the host's EFER.NX.  Not only does KVM context switch EFER if
> host.EFER.NX=1 && guest.EFER.NX=0, KVM also forces host.EFER.NX=0 &&
> guest.EFER.NX=1 when using shadow paging (to emulate SMEP).  Furthermore,
> the entire motivation for the restriction was made obsolete over a decade
> ago when Intel added dedicated host and guest EFER fields in the VMCS
> (Nehalem timeframe), which reduced the overhead of context switching EFER
> from 400+ cycles (2 * WRMSR + 1 * RDMSR) to a mere ~2 cycles.

Both pretty good points. :)  Queued, thanks.

Paolo


      reply	other threads:[~2021-08-06 11:58 UTC|newest]

Thread overview: 2+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-08-05 18:38 Sean Christopherson
2021-08-06 11:58 ` Paolo Bonzini [this message]

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