On Wed, 2023-04-26 at 13:13 +0800, Binbin Wu wrote: > > On 4/26/2023 11:05 AM, Chao Gao wrote: > > On Wed, Apr 26, 2023 at 06:48:21AM +0800, Huang, Kai wrote: > > > ... when EPT is on, as you mentioned guest can update CR3 w/o causing VMEXIT to > > > KVM. > > > > > > Is there any global enabling bit in any of CR to turn on/off LAM globally? It > > > seems there isn't because AFAICT the bits in CR4 are used to control super mode > > > linear address but not LAM in global? > > Right. > > > > > So if it is true, then it appears hardware depends on CPUID purely to decide > > > whether to perform LAM or not. > > > > > > Which means, IIRC, when EPT is on, if we don't expose LAM to the guest on the > > > hardware that supports LAM, I think guest can still enable LAM in CR3 w/o > > > causing any trouble (because the hardware actually supports this feature)? > > Yes. But I think it is a non-issue ... > > > > > If it's true, it seems we should trap CR3 (at least loading) when hardware > > > supports LAM but it's not exposed to the guest, so that KVM can correctly reject > > > any LAM control bits when guest illegally does so? > > > > > Other features which need no explicit enablement (like AVX and other > > new instructions) have the same problem. > > > > The impact is some guests can use features which they are not supposed > > to use. Then they might be broken after migration or kvm's instruction > > emulation. But they put themselves at stake, KVM shouldn't be blamed. > Agree. > > > > > The downside of intercepting CR3 is the performance impact on existing > > VMs (all with old CPU models and thus all have no LAM). If they are > > migrated to LAM-capable parts in the future, they will suffer > > performance drop even though they are good tenents (i.e., won't try to > > use LAM). > > > > IMO, the value of preventing some guests from setting LAM_U48/U57 in CR3 > > when EPT=on cannot outweigh the performance impact. So, I vote to > > document in changelog or comments that: > > A guest can enable LAM for userspace pointers when EPT=on even if LAM > > isn't exposed to it. KVM doens't prevent this out of performance > > consideration > > How about add the comments on the code: > > + /* > + * A guest can enable LAM for userspace pointers when EPT=on on a > + * processor supporting LAM even if LAM isn't exposed to it. > + * KVM doesn't prevent this out of performance considerations. > + */ > if (guest_cpuid_has(vcpu, X86_FEATURE_LAM)) > vcpu->arch.cr3_ctrl_bits |= X86_CR3_LAM_U48 | > X86_CR3_LAM_U57; > > I would say we should at least call out in the changelog, but I don't have opinion on whether to have this comment around this code or not.
