Re: [PATCH 02/19] x86/fpu: Prepare KVM for dynamically enabled states

[Paolo Bonzini <pbonzini@redhat.com>]

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