From: Vitaly Kuznetsov <firstname.lastname@example.org> To: Jim Mattson <email@example.com>, Paolo Bonzini <firstname.lastname@example.org> Cc: "kvm list" <email@example.com>, "Radim Krčmář" <firstname.lastname@example.org>, "Liran Alon" <email@example.com>, LKML <firstname.lastname@example.org> Subject: Re: [PATCH] x86/kvm/nVMX: tweak shadow fields Date: Mon, 12 Nov 2018 15:39:40 +0100 [thread overview] Message-ID: <email@example.com> (raw) In-Reply-To: <CALMp9eQ4493qH4sA4e5UibaHhatEKnztGNBO2s40jc2jXXCdUQ@mail.gmail.com> Jim Mattson <firstname.lastname@example.org> writes: > I'm not convinced that the "one size fits all" and "context-free" > approaches to VMCS shadowing are terribly effective. > > For example, we never shadow VMX_INSTRUCTION_INFO, but if we just > reflected an exit to L1 for which that field is defined, there's > probably a good chance that L1 will use it. We always shadow > VM_EXIT_INTR_INFO, but if we didn't just reflect exit reason 0 to L1, > it's not likely to be read. If the L2 guest is in legacy mode or > compatibility mode, L1 is much more likely to be interested in the > contents of the descriptor cache than if the guest is in 64-bit mode. > > Some hypervisors write TSC_OFFSET quite frequently. Others rarely. > Last time I checked (it's been a while), VirtualBox was always > interested in everything. :-) Kvm, Hyper-V, VMware, VirtualBox, > Parallels...they all have different patterns, and they change from > release to release. > > 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 haven't thought this through, especially read/write shadowing, but we can probably start with an empty bitmap and later shadow it when we get over some threshold of vmread/vmwrite exits we enabling shadowing. The question is when we un-shadow it. For example, we can un-shadow a field for writing every time we see it was not changed between two exits to L0 (so we're trying to write the same value to vmcs12). -- Vitaly
next prev parent reply other threads:[~2018-11-12 14:39 UTC|newest] Thread overview: 5+ messages / expand[flat|nested] mbox.gz Atom feed top 2018-10-19 14:16 Vitaly Kuznetsov 2018-10-19 16:45 ` Paolo Bonzini 2018-11-09 22:11 ` Jim Mattson 2018-11-12 14:39 ` Vitaly Kuznetsov [this message] 2018-11-14 11:34 ` Paolo Bonzini
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