Messages in this thread | | | Date | Mon, 13 Dec 2021 13:45:49 +0100 | Subject | Re: [PATCH 02/19] x86/fpu: Prepare KVM for dynamically enabled states | From | Paolo Bonzini <> |
| |
On 12/13/21 13:00, Thomas Gleixner wrote: > On Mon, Dec 13 2021 at 10:12, Paolo Bonzini wrote: >> On 12/8/21 01:03, Yang Zhong wrote: >>> - user_xfeatures >>> >>> Track which features are currently enabled for the vCPU >> >> Please rename to alloc_xfeatures > > That name makes no sense at all. This has nothing to do with alloc.
Isn't that the features for which space is currently allocated?
fpstate_realloc does
+ if (guest_fpu) { + newfps->is_guest = true; + newfps->is_confidential = curfps->is_confidential; + guest_fpu->user_xfeatures |= xfeatures; + } +
and kvm_check_guest_realloc_fpstate does
+ if ((guest_fpu->user_xfeatures & request) != request) { + vcpu->arch.guest_fpu.realloc_request |= request; + return true; + }
Reading "user_xfeatures" in there is cryptic, it seems like it's something related to the userspace thread or group that has invoked the KVM ioctl. If it's renamed to alloc_xfeatures, then this:
+ missing = request & ~guest_fpu->alloc_xfeatures; + if (missing) { + vcpu->arch.guest_fpu.realloc_request |= missing; + return true; + }
makes it obvious that the allocation is for features that are requested but haven't been allocated in the xstate yet.
>>> - user_perm >>> >>> Copied from guest_perm of the group leader thread. The first >>> vCPU which does the copy locks the guest_perm >> >> Please rename to perm_xfeatures. > > All of that is following the naming conventions in the FPU code related > to permissions etc.
perm or guest_perm is just fine as well; but user_perm is not (there's no preexisting reference to user_perm in the code, in fact, as far as I can see).
>>> - realloc_request >>> >>> KVM sets this field to request dynamically-enabled features >>> which require reallocation of @fpstate >> >> This field should be in vcpu->arch, and there is no need for >> fpu_guest_realloc_fpstate. Rename __xfd_enable_feature to >> fpu_enable_xfd_feature and add it to the public API, then just do >> >> if (unlikely(vcpu->arch.xfd_realloc_request)) { >> u64 request = vcpu->arch.xfd_realloc_request; >> ret = fpu_enable_xfd(request, enter_guest); >> } >> >> to kvm_put_guest_fpu. > > Why? Yet another export of FPU internals just because?
It's one function more and one field less. I prefer another export of FPU internals, to a write to a random field with undocumented invariants.
For example, why WARN_ON_ONCE if enter_guest == true? If you enter the guest after the host has restored MSR_IA32_XFD with KVM_SET_MSR, the WARN_ON_ONCE can actually fire. It would be just fine to skip the reallocation until enter_guest == false, which is you actually XSAVE into the guest_fpu.
As an aside, realloc_request (if it survives, see below) shouldn't be added until patch 6.
> Also what clears the reallocation request and what is the @enter_guest > argument supposed to help with?
Nothing---make it
if (unlikely(vcpu->arch.xfd_realloc_request)) { u64 request = vcpu->arch.xfd_realloc_request; ret = fpu_enable_xfd(request, &vcpu->arch.guest_fpu); if (!ret) vcpu->arch.xfd_realloc_request = 0; }
but in fact, I'm not sure why the request has to be delayed at all. The obvious implementation of a write to XFD, after all the validity checks, is just
/* This function just calls xfd_enable_feature. */ r = fpu_guest_alloc_xfeatures(&vcpu->arch.guest_fpu, vcpu->arch.xcr0 & ~xfd); /* * An error means that userspace has screwed up by not doing * arch_prctl(ARCH_REQ_XCOMP_GUEST_PERM). If we are here coming * from a ioctl fail it, if the guest has done WRMSR or XSETBV * it will inject a #GP. */ if (r < 0) return 1;
vcpu->arch.xfd = xfd;
with none of the kvm_check_guest_realloc_fpstate or KVM_EXIT_FPU_REALLOC business. No field and a simple, self-contained external API. The same code works for __kvm_set_xcr as well, just with "xcr0 & ~vcpu->arch.xfd" as the second argument instead.
(Maybe I'm missing why KVM_EXIT_FPU_REALLOC is needed, but I'm not ashamed to say that, given that new userspace API was added with documentation or tests. The only comment in the code is:
+ /* + * Check if fpstate reallocate is required. If yes, then + * let the fpu core do reallocation and update xfd; + * otherwise, update xfd here. + */ + if (kvm_check_guest_realloc_fpstate(vcpu, data)) { + vcpu->run->exit_reason = KVM_EXIT_FPU_REALLOC; + vcpu->arch.complete_userspace_io = + kvm_skip_emulated_instruction; + return KVM_MSR_RET_USERSPACE; + } +
which has nothing to do with the actual content of either kvm_check_guest_realloc_fpstate or the "then" branch. Userspace can just do ARCH_REQ_XCOMP_GUEST_PERM based on the guest CPUID, before KVM_SET_CPUID2, KVM_SET_MSR or KVM_SET_XCR).
Paolo
| |