From: Paolo Bonzini <email@example.com> To: Vitaly Kuznetsov <firstname.lastname@example.org>, Jim Mattson <email@example.com> Cc: "kvm list" <firstname.lastname@example.org>, "Radim Krčmář" <email@example.com>, "Liran Alon" <firstname.lastname@example.org>, LKML <email@example.com> Subject: Re: [PATCH] x86/kvm/nVMX: tweak shadow fields Date: Wed, 14 Nov 2018 12:34:36 +0100 [thread overview] Message-ID: <firstname.lastname@example.org> (raw) In-Reply-To: <email@example.com> 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
prev parent reply other threads:[~2018-11-14 11:34 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 2018-11-14 11:34 ` Paolo Bonzini [this message]
Reply instructions: You may reply publicly to this message via plain-text email using any one of the following methods: * Save the following mbox file, import it into your mail client, and reply-to-all from there: mbox Avoid top-posting and favor interleaved quoting: https://en.wikipedia.org/wiki/Posting_style#Interleaved_style * Reply using the --to, --cc, and --in-reply-to switches of git-send-email(1): git send-email \ --firstname.lastname@example.org \ --email@example.com \ --firstname.lastname@example.org \ --email@example.com \ --firstname.lastname@example.org \ --email@example.com \ --firstname.lastname@example.org \ --email@example.com \ --subject='Re: [PATCH] x86/kvm/nVMX: tweak shadow fields' \ /path/to/YOUR_REPLY https://kernel.org/pub/software/scm/git/docs/git-send-email.html * If your mail client supports setting the In-Reply-To header via mailto: links, try the mailto: link
This is a public inbox, see mirroring instructions for how to clone and mirror all data and code used for this inbox; as well as URLs for NNTP newsgroup(s).