From: Borislav Petkov <bp@alien8.de>
To: Sean Christopherson <seanjc@google.com>
Cc: Kai Huang <kai.huang@intel.com>,
Dave Hansen <dave.hansen@intel.com>,
linux-sgx@vger.kernel.org, kvm@vger.kernel.org, x86@kernel.org,
jarkko@kernel.org, luto@kernel.org, haitao.huang@intel.com,
pbonzini@redhat.com, tglx@linutronix.de, mingo@redhat.com,
hpa@zytor.com
Subject: Re: [RFC PATCH 04/23] x86/cpufeatures: Add SGX1 and SGX2 sub-features
Date: Mon, 11 Jan 2021 20:09:01 +0100 [thread overview]
Message-ID: <20210111190901.GG25645@zn.tnic> (raw)
In-Reply-To: <X/yQyUx4+veuSO0e@google.com>
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.
https://people.kernel.org/tglx/notes-about-netiquette
next prev parent reply other threads:[~2021-01-11 19:09 UTC|newest]
Thread overview: 111+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-01-06 1:55 [RFC PATCH 00/23] KVM SGX virtualization support Kai Huang
2021-01-06 1:55 ` [RFC PATCH 01/23] x86/sgx: Split out adding EPC page to free list to separate helper Kai Huang
2021-01-11 22:38 ` Jarkko Sakkinen
2021-01-12 0:19 ` Kai Huang
2021-01-12 21:45 ` Sean Christopherson
2021-01-13 1:15 ` Kai Huang
2021-01-13 17:05 ` Jarkko Sakkinen
2021-01-06 1:55 ` [RFC PATCH 02/23] x86/sgx: Add enum for SGX_CHILD_PRESENT error code Kai Huang
2021-01-06 18:28 ` Dave Hansen
2021-01-06 21:40 ` Kai Huang
2021-01-12 0:26 ` Jarkko Sakkinen
2021-01-11 23:32 ` Jarkko Sakkinen
2021-01-12 0:16 ` Kai Huang
2021-01-12 1:46 ` Jarkko Sakkinen
2021-01-06 1:55 ` [RFC PATCH 03/23] x86/sgx: Introduce virtual EPC for use by KVM guests Kai Huang
2021-01-06 19:35 ` Dave Hansen
2021-01-06 20:35 ` Sean Christopherson
2021-01-07 0:47 ` Kai Huang
2021-01-07 0:52 ` Dave Hansen
2021-01-07 1:38 ` Kai Huang
2021-01-07 5:00 ` Dave Hansen
2021-01-07 1:42 ` Kai Huang
2021-01-07 5:02 ` Dave Hansen
2021-01-15 14:07 ` Kai Huang
2021-01-15 15:39 ` Dave Hansen
2021-01-15 21:33 ` Kai Huang
2021-01-15 21:45 ` Sean Christopherson
2021-01-15 22:30 ` Kai Huang
2021-01-11 23:38 ` Jarkko Sakkinen
2021-01-12 0:56 ` Kai Huang
2021-01-12 1:50 ` Jarkko Sakkinen
2021-01-12 2:03 ` Kai Huang
2021-01-06 1:55 ` [RFC PATCH 04/23] x86/cpufeatures: Add SGX1 and SGX2 sub-features Kai Huang
2021-01-06 19:39 ` Dave Hansen
2021-01-06 22:12 ` Kai Huang
2021-01-06 22:21 ` Dave Hansen
2021-01-06 22:56 ` Kai Huang
2021-01-06 23:19 ` Sean Christopherson
2021-01-06 23:33 ` Dave Hansen
2021-01-06 23:56 ` Kai Huang
2021-01-06 23:40 ` Kai Huang
2021-01-06 23:43 ` Dave Hansen
2021-01-06 23:56 ` Kai Huang
2021-01-06 22:15 ` Borislav Petkov
2021-01-06 23:09 ` Kai Huang
2021-01-07 6:41 ` Borislav Petkov
2021-01-08 2:00 ` Kai Huang
2021-01-08 5:10 ` Dave Hansen
2021-01-08 7:03 ` Kai Huang
2021-01-08 7:17 ` Borislav Petkov
2021-01-08 8:06 ` Kai Huang
2021-01-08 8:13 ` Borislav Petkov
2021-01-08 9:00 ` Kai Huang
2021-01-08 23:55 ` Sean Christopherson
2021-01-09 0:35 ` Borislav Petkov
2021-01-09 1:01 ` Sean Christopherson
2021-01-09 1:19 ` Borislav Petkov
2021-01-11 17:54 ` Sean Christopherson
2021-01-11 19:09 ` Borislav Petkov [this message]
2021-01-11 19:20 ` Sean Christopherson
2021-01-12 2:01 ` Kai Huang
2021-01-12 12:13 ` Borislav Petkov
2021-01-12 17:15 ` Sean Christopherson
2021-01-12 17:51 ` Borislav Petkov
2021-01-12 21:07 ` Kai Huang
2021-01-12 23:17 ` Sean Christopherson
2021-01-13 1:05 ` Kai Huang
2021-01-11 23:39 ` Jarkko Sakkinen
2021-01-06 1:55 ` [RFC PATCH 05/23] x86/cpu/intel: Allow SGX virtualization without Launch Control support Kai Huang
2021-01-06 19:54 ` Dave Hansen
2021-01-06 22:34 ` Kai Huang
2021-01-06 22:38 ` Dave Hansen
2021-01-06 1:56 ` [RFC PATCH 06/23] x86/sgx: Expose SGX architectural definitions to the kernel Kai Huang
2021-01-06 1:56 ` [RFC PATCH 07/23] x86/sgx: Move ENCLS leaf definitions to sgx_arch.h Kai Huang
2021-01-06 1:56 ` [RFC PATCH 08/23] x86/sgx: Add SGX2 ENCLS leaf definitions (EAUG, EMODPR and EMODT) Kai Huang
2021-01-06 1:56 ` [RFC PATCH 09/23] x86/sgx: Add encls_faulted() helper Kai Huang
2021-01-06 1:56 ` [RFC PATCH 10/23] x86/sgx: Add helper to update SGX_LEPUBKEYHASHn MSRs Kai Huang
2021-01-06 19:56 ` Dave Hansen
2021-01-06 1:56 ` [RFC PATCH 11/23] x86/sgx: Add helpers to expose ECREATE and EINIT to KVM Kai Huang
2021-01-06 20:12 ` Dave Hansen
2021-01-06 21:04 ` Sean Christopherson
2021-01-06 21:23 ` Dave Hansen
2021-01-06 22:58 ` Kai Huang
2021-01-06 1:56 ` [RFC PATCH 12/23] x86/sgx: Move provisioning device creation out of SGX driver Kai Huang
2021-01-06 1:56 ` [RFC PATCH 13/23] KVM: VMX: Convert vcpu_vmx.exit_reason to a union Kai Huang
2021-01-06 1:56 ` [RFC PATCH 14/23] KVM: x86: Export kvm_mmu_gva_to_gpa_{read,write}() for SGX (VMX) Kai Huang
2021-01-06 1:56 ` [RFC PATCH 15/23] KVM: x86: Define new #PF SGX error code bit Kai Huang
2021-01-06 1:56 ` [RFC PATCH 16/23] KVM: x86: Add SGX feature leaf to reverse CPUID lookup Kai Huang
2021-01-06 1:56 ` [RFC PATCH 17/23] KVM: VMX: Add basic handling of VM-Exit from SGX enclave Kai Huang
2021-01-06 1:56 ` [RFC PATCH 18/23] KVM: VMX: Frame in ENCLS handler for SGX virtualization Kai Huang
2021-01-06 1:56 ` [RFC PATCH 19/23] KVM: VMX: Add SGX ENCLS[ECREATE] handler to enforce CPUID restrictions Kai Huang
2021-01-06 1:56 ` [RFC PATCH 20/23] KVM: VMX: Add emulation of SGX Launch Control LE hash MSRs Kai Huang
2021-01-06 1:56 ` [RFC PATCH 21/23] KVM: VMX: Add ENCLS[EINIT] handler to support SGX Launch Control (LC) Kai Huang
2021-01-06 1:56 ` [RFC PATCH 22/23] KVM: VMX: Enable SGX virtualization for SGX1, SGX2 and LC Kai Huang
2021-01-06 1:58 ` [RFC PATCH 23/23] KVM: x86: Add capability to grant VM access to privileged SGX attribute Kai Huang
2021-01-06 2:22 ` [RFC PATCH 00/23] KVM SGX virtualization support Kai Huang
2021-01-06 17:07 ` Dave Hansen
2021-01-07 0:34 ` Kai Huang
2021-01-07 0:48 ` Dave Hansen
2021-01-07 1:50 ` Kai Huang
2021-01-07 16:14 ` Sean Christopherson
2021-01-08 2:16 ` Kai Huang
2021-01-11 17:20 ` Jarkko Sakkinen
2021-01-11 18:37 ` Sean Christopherson
2021-01-12 1:58 ` Jarkko Sakkinen
2021-01-12 1:14 ` Kai Huang
2021-01-12 2:02 ` Jarkko Sakkinen
2021-01-12 2:07 ` Kai Huang
2021-01-15 14:43 ` Kai Huang
2021-01-16 9:31 ` Jarkko Sakkinen
2021-01-16 9:50 ` Jarkko Sakkinen
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