From: Paolo Bonzini <pbonzini@redhat.com>
To: Jim Mattson <jmattson@google.com>,
Sean Christopherson <sean.j.christopherson@intel.com>
Cc: "Radim Krčmář" <rkrcmar@redhat.com>,
"kvm list" <kvm@vger.kernel.org>,
"Liran Alon" <liran.alon@oracle.com>
Subject: Re: [PATCH 1/7] KVM: nVMX: Intercept VMWRITEs to read-only shadow VMCS fields
Date: Thu, 13 Jun 2019 19:18:02 +0200 [thread overview]
Message-ID: <9d82caf7-1735-a5e8-8206-bdec3ddf12d4@redhat.com> (raw)
In-Reply-To: <CALMp9eRb8GC1NH9agiWWwkY5ac4CKxZqzobzmLiV5FiscV_B+A@mail.gmail.com>
On 13/06/19 19:02, Jim Mattson wrote:
> On Tue, May 7, 2019 at 8:36 AM Sean Christopherson
> <sean.j.christopherson@intel.com> wrote:
>
>> Not intercepting fields tagged read-only also allows for additional
>> optimizations, e.g. marking GUEST_{CS,SS}_AR_BYTES as SHADOW_FIELD_RO
>> since those fields are rarely written by a VMMs, but read frequently.
>
> Do you have data to support this, or is this just a gut feeling? The
> last time I looked at Virtual Box (which was admittedly a long time
> ago), it liked to read and write just about every VMCS guest-state
> field it could find on every VM-exit.
I have never looked at VirtualBox, but most other hypervisors do have a
common set of fields (give or take a couple) that they like to read
and/or write on most if not every vmexit.
Also, while this may vary dynamically based on the L2 guest that is
running, this is much less true for unrestricted-guest processors.
Without data on _which_ scenarios are bad for a static set of shadowed
fields, I'm not really happy to add even more complexity.
Paolo
> The decision of which fields to shadow is really something that should
> be done dynamically, depending on the behavior of the guest hypervisor
> (which may vary depending on the L2 guest it's running!) Making the
> decision statically is bound to result in a poor outcome for some
> scenarios.
next prev parent reply other threads:[~2019-06-13 17:18 UTC|newest]
Thread overview: 15+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-05-07 15:36 [PATCH 0/7] KVM: nVMX: Optimize VMCS data copying Sean Christopherson
2019-05-07 15:36 ` [PATCH 1/7] KVM: nVMX: Intercept VMWRITEs to read-only shadow VMCS fields Sean Christopherson
2019-06-06 13:26 ` Paolo Bonzini
2019-06-13 17:02 ` Jim Mattson
2019-06-13 17:18 ` Paolo Bonzini [this message]
2019-06-13 17:36 ` Jim Mattson
2019-06-13 17:59 ` Paolo Bonzini
2019-05-07 15:36 ` [PATCH 2/7] KVM: nVMX: Intercept VMWRITEs to GUEST_{CS,SS}_AR_BYTES Sean Christopherson
2019-06-06 13:31 ` Paolo Bonzini
2019-05-07 15:36 ` [PATCH 3/7] KVM: nVMX: Track vmcs12 offsets for shadowed VMCS fields Sean Christopherson
2019-05-07 15:36 ` [PATCH 4/7] KVM: nVMX: Lift sync_vmcs12() out of prepare_vmcs12() Sean Christopherson
2019-05-07 15:36 ` [PATCH 5/7] KVM: nVMX: Use descriptive names for VMCS sync functions and flags Sean Christopherson
2019-05-07 15:36 ` [PATCH 6/7] KVM: nVMX: Add helpers to identify shadowed VMCS fields Sean Christopherson
2019-05-07 15:36 ` [PATCH 7/7] KVM: nVMX: Sync rarely accessed guest fields only when needed Sean Christopherson
2019-06-06 15:29 ` Paolo Bonzini
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