From: Sean Christopherson <seanjc@google.com>
To: Jing Liu <jing2.liu@linux.intel.com>
Cc: pbonzini@redhat.com, kvm@vger.kernel.org,
linux-kernel@vger.kernel.org, jing2.liu@intel.com
Subject: Re: [PATCH RFC 2/7] kvm: x86: Introduce XFD MSRs as passthrough to guest
Date: Mon, 24 May 2021 21:43:00 +0000 [thread overview]
Message-ID: <YKwd5OTXr97Fxfok@google.com> (raw)
In-Reply-To: <20210207154256.52850-3-jing2.liu@linux.intel.com>
On Sun, Feb 07, 2021, Jing Liu wrote:
> Passthrough both MSRs to let guest access and write without vmexit.
Why? Except for read-only MSRs, e.g. MSR_CORE_C1_RES, passthrough MSRs are
costly to support because KVM must context switch the MSR (which, by the by, is
completely missing from the patch).
In other words, if these MSRs are full RW passthrough, guests with XFD enabled
will need to load the guest value on entry, save the guest value on exit, and
load the host value on exit. That's in the neighborhood of a 40% increase in
latency for a single VM-Enter/VM-Exit roundtrip (~1500 cycles => >2000 cycles).
I'm not saying these can't be passhthrough, but there needs to be strong
justification for letting the guest read/write them directly.
next prev parent reply other threads:[~2021-05-24 21:43 UTC|newest]
Thread overview: 39+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-02-07 15:42 [PATCH RFC 0/7] Introduce support for guest AMX feature Jing Liu
2021-02-07 15:42 ` [PATCH RFC 1/7] kvm: x86: Expose XFD CPUID to guest Jing Liu
2021-05-24 21:34 ` Sean Christopherson
2021-06-07 3:27 ` Liu, Jing2
2021-02-07 15:42 ` [PATCH RFC 2/7] kvm: x86: Introduce XFD MSRs as passthrough " Jing Liu
2021-05-24 21:43 ` Sean Christopherson [this message]
2021-05-24 21:57 ` Jim Mattson
2021-06-02 3:12 ` Liu, Jing2
2021-06-23 17:50 ` Dave Hansen
2021-06-28 2:00 ` Liu, Jing2
2021-06-29 17:58 ` Dave Hansen
2021-07-06 7:33 ` Liu, Jing2
2021-02-07 15:42 ` [PATCH RFC 3/7] kvm: x86: XSAVE state and XFD MSRs context switch Jing Liu
2021-02-07 11:49 ` Borislav Petkov
2021-02-08 3:35 ` Liu, Jing2
2021-02-08 10:25 ` Paolo Bonzini
2021-02-08 17:31 ` Sean Christopherson
2021-02-08 17:45 ` Paolo Bonzini
2021-02-08 18:04 ` Sean Christopherson
2021-02-08 18:12 ` Paolo Bonzini
2021-02-08 18:55 ` Konrad Rzeszutek Wilk
2021-02-22 8:51 ` Liu, Jing2
2021-02-22 8:36 ` Liu, Jing2
2021-02-07 15:42 ` [PATCH RFC 4/7] kvm: x86: Add new ioctls for XSAVE extension Jing Liu
2021-05-24 21:50 ` Sean Christopherson
2021-05-26 6:09 ` Liu, Jing2
2021-05-26 14:43 ` Sean Christopherson
2021-06-01 10:24 ` Liu, Jing2
2021-06-07 5:23 ` Liu, Jing2
2021-05-24 22:06 ` Jim Mattson
2021-05-26 6:11 ` Liu, Jing2
2021-02-07 15:42 ` [PATCH RFC 5/7] kvm: x86: Revise CPUID.D.1.EBX for alignment rule Jing Liu
2021-05-24 21:28 ` Sean Christopherson
2021-06-03 4:45 ` Liu, Jing2
2021-02-07 15:42 ` [PATCH RFC 6/7] kvm: x86: Add AMX_TILE, AMX_INT8 and AMX_BF16 support Jing Liu
2021-02-07 15:42 ` [PATCH RFC 7/7] kvm: x86: AMX XCR0 support for guest Jing Liu
2021-05-24 21:53 ` Sean Christopherson
2021-05-26 7:54 ` Liu, Jing2
2021-05-26 14:54 ` Sean Christopherson
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