linux-kernel - Re: [PATCH] x86/kvm/nVMX: tweak shadow fields

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Date:   Wed, 14 Nov 2018 12:34:36 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Vitaly Kuznetsov <vkuznets@...hat.com>,
        Jim Mattson <jmattson@...gle.com>
Cc:     kvm list <kvm@...r.kernel.org>,
        Radim Krčmář <rkrcmar@...hat.com>,
        Liran Alon <liran.alon@...cle.com>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] x86/kvm/nVMX: tweak shadow fields

On 12/11/2018 15:39, Vitaly Kuznetsov wrote:
>> Is it worth having a set of VMCS shadowing bitmaps per-vCPU, in order
>> to make better use of this feature?
> Per CPU or not, to improve the feature we'll probably need some sort of
> an 'adaptive' algorithm picking which fields to shadow. 

I agree, making it per-VCPU is not useful alone.  The question is to
balance.  The complexity and the number of fields that have to be copied
between the VMCSes.

If a vmexit type is rare, it makes sense not to shadow a field that
would be always defined by that vmexit type, rather than pay a fixed
price (even if it is loop overhead only) on all vmexits; this is the
case VMX_INSTRUCTION_INFO.

One thing that would make sense is to have separate shadow bitmaps for
32- and 64-bit L2.  32-bit L2 probably will need to shadow at least the
segment bases.  But for 64-bit L2, the current set is small and nice.

There are still a few things that can be refined, but it's small things:

1) EXCEPTION_BITMAP which can go because everyone is probably using
eager FPU these days---and has always been if you have shadow VMCS;

2) CR0_READ_SHADOW/CR4_READ_SHADOW/GUEST_CR0/GUEST_CR4 were needed on
old KVM and would need to be tested on other hypervisors, but are
probably unnecessary;

3) I would be surprised if HOST_FS_BASE/HOST_GS_BASE are needed too,
though again you'd need testing on other hypervisors

Overall, I prefer simple code that optimizes the common case very well,
rather than complex code that tries to cover all bases...

Paolo