Re: [RFC PATCH 04/23] x86/cpufeatures: Add SGX1 and SGX2 sub-features

Borislav Petkov
2021-01-11T19:09:58Z

On Mon, Jan 11, 2021 at 09:54:17AM -0800, Sean Christopherson wrote:
> Yes, but KVM including the bug caps in kvm_cpu_caps is extremely unlikely, and
> arguably flat out wrong. Currently, kvm_cpu_caps includes only CPUID-based
> features that can be exposed direcly to the guest. I could see a scenario where
> KVM exposed "bug" capabilities to the guest via a paravirt interface, but I
> would expect that KVM would either filter and expose the kernel's bug caps
> without userspace input, or would add a KVM-defined paravirt CPUID leaf to
> enumerate the caps and track _that_ in kvm_cpu_caps.
>
> Anyways, I agree that overlapping the bug caps it's a bit of unnecessary
> cleverness. I'm not opposed to incorporating NBUGINTS into KVM, but that would
> mean explicitly pulling in even more x86_capability implementation details.

Also, the kernel and kvm being part of it :) kinda tries to fix those
bugs and not expose them to the guest so exposing a bug would probably
be only for testing purposes...

> That part is deliberate and isn't a dependency so much as how things are
> implemented. The true dependency is on the bit offsets within each word.

Right.

> The kernel could completely rescramble the word numbering and KVM
> would chug along happily. What KVM won't play nice with is if the
> kernel broke up a hardware- defined, gathered CPUID leaf/word into
> scattered features spread out amongst multiple Linux-defined words.

Yes, kvm wants the bits just as they are in the CPUID leafs from the hw.

> It's mostly historical; before the kvm_cpu_caps concept was introduced, the code
> had grown organically to include both boot_cpu_data and raw CPUID info. The
> vast, vast majority of the time, doing CPUID is likely redundant. But, as noted
> in commit d8577a4c238f ("KVM: x86: Do host CPUID at load time to mask KVM cpu
> caps"), the code is quite cheap and runs once at KVM load. My argument back
> then was, and still is, that an extra bit of paranoia is justified since the
> code and operations are quite nearly free.

Ok.

> This particular dependency can be broken, and quite easily at that. Rather than
> memcpy() boot_cpu_data.x86_capability, it's trivially easy to redefine the F()
> macro to invoke boot_cpu_has(), which would allow dropping the memcpy(). The
> big downside, and why I didn't post the code, is that doing so means every
> feature routed through F() requires some form of BT+Jcc (or CMOVcc) sequence,
> whereas the mempcy() approach allows the F() features to be encoded as a single
> literal by the compiler.
>
> From a latency perspective, the extra code is negligible. The big issue is that
> all those extra checks add 2k+ bytes of code. Eliminating the mempcy() doesn't
> actually break KVM's dependency on the bit offsets, so we'd be bloating kvm.ko
> by a noticeable amount without providing substantial value.
>
> And, this behavior is mostly opportunistic; the true justification/motiviation
> for taking a dependency on the X86_FEATURE_* bit offsets is for communication
> with userspace, querying the guest CPU model, and runtime checks.

Ok, I guess we'll try to find a middle ground here and not let stuff
grow too ugly to live.

> It's effectively for communication with userspace. Userspace, via ioctl(),
> dictates the vCPU model to KVM, including the exact CPUID results.

And using the CPUID leafs with the exact bit positions is sort of an
"interface" there, I see.

> to properly
> virtualize/emulate the defined vCPU model, KVM must query the dictated CPUID
> results to determine what features are supported, what guest operations
> should fault, etc... E.g. if the vCPU model, via CPUID, states that SMEP isn't
> supported then KVM needs to inject a #GP if the guest attempts to set CR4.SMEP.
>
> KVM also uses the hardware-defined CPUID ABI to advertise which features are
> supported by both hardware and KVM. This is the kvm_cpu_cap stuff, where KVM
> reads boot_cpu_data to see what features were enabled by the kernel.

Right.

> It would be possible for KVM to break the dependency on X86_FEATURE_* bit
> offsets by defining a translation layer, but I strongly feel that adding manual
> translations will do more harm than good as it increases the odds of us botching
> a translation or using the wrong feature flag, creates potential namespace
> conflicts, etc...

Ok, lemme see if we might encounter more issues down the road...

+enum kvm_only_cpuid_leafs {
+ CPUID_12_EAX = NCAPINTS,
+ NR_KVM_CPU_CAPS,
+
+ NKVMCAPINTS = NR_KVM_CPU_CAPS - NCAPINTS,
+};
+

What happens when we decide to allocate a separate leaf for CPUID_12_EAX
down the road?

You do it already here

Subject: [PATCH 04/13] x86/cpufeatures: Assign dedicated feature word for AMD mem encryption

for the AMD leaf.

I'm thinking this way around - from scattered to a hw one - should be ok
because that should work easily. The other way around, taking a hw leaf
and scattering it around x86_capability[] array elems would probably be
nasty but with your change that should work too.

Yah, I'm just hypothesizing here - I don't think this "other way around"
will ever happen...

Hmm, yap, I can cautiously say that with your change we should be ok...

Thx.

--
Regards/Gruss,
Boris.