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Message-ID: <cf97cfba-941a-5a77-6591-fa84ef6fe8d1@redhat.com>
Date: Fri, 4 Aug 2023 23:32:00 +0200
From: Paolo Bonzini <pbonzini@...hat.com>
To: Sean Christopherson <seanjc@...gle.com>,
Weijiang Yang <weijiang.yang@...el.com>
Cc: Chao Gao <chao.gao@...el.com>, peterz@...radead.org,
john.allen@....com, kvm@...r.kernel.org,
linux-kernel@...r.kernel.org, rick.p.edgecombe@...el.com,
binbin.wu@...ux.intel.com
Subject: Re: [PATCH v5 09/19] KVM:x86: Make guest supervisor states as
non-XSAVE managed
On 8/4/23 22:45, Sean Christopherson wrote:
>>>> +void save_cet_supervisor_ssp(struct kvm_vcpu *vcpu)
>>>> +{
>>>> + if (unlikely(guest_can_use(vcpu, X86_FEATURE_SHSTK))) {
> Drop the unlikely, KVM should not speculate on the guest configuration or underlying
> hardware.
In general unlikely() can still be a good idea if you have a fast path
vs. a slow path; the extra cost of a branch will be much more visible on
the fast path. That said the compiler should already be doing that.
> the Pros:
> - Super easy to implement for KVM.
> - Automatically avoids saving and restoring this data when the vmexit
> is handled within KVM.
>
> the Cons:
> - Unnecessarily restores XFEATURE_CET_KERNEL when switching to
> non-KVM task's userspace.
> - Forces allocating space for this state on all tasks, whether or not
> they use KVM, and with likely zero users today and the near future.
> - Complicates the FPU optimization thinking by including things that
> can have no affect on userspace in the FPU
I'm not sure if Linux will ever use XFEATURE_CET_KERNEL. Linux does not
use MSR_IA32_PL{1,2}_SSP; MSR_IA32_PL0_SSP probably would be per-CPU but
it is not used while in ring 0 (except for SETSSBSY) and the restore can
be delayed until return to userspace. It is not unlike the SYSCALL MSRs.
So I would treat the bit similar to the dynamic features even if it's
not guarded by XFD, for example
#define XFEATURE_MASK_USER_DYNAMIC XFEATURE_MASK_XTILE_DATA
#define XFEATURE_MASK_USER_OPTIONAL \
(XFEATURE_MASK_DYNAMIC | XFEATURE_MASK_CET_KERNEL)
where XFEATURE_MASK_USER_DYNAMIC is used for xfd-related tasks but
everything else uses XFEATURE_MASK_USER_OPTIONAL.
Then you'd enable the feature by hand when allocating the guest fpstate.
> Especially because another big negative is that not utilizing XSTATE bleeds into
> KVM's ABI. Userspace has to be told to manually save+restore MSRs instead of just
> letting KVM_{G,S}ET_XSAVE handle the state. And that will create a bit of a
> snafu if Linux does gain support for SSS.
I don't think this matters, we don't have any MSRs in KVM_GET/SET_XSAVE
and in fact we can't even add them since the uABI uses the non-compacted
format. MSRs should be retrieved and set via KVM_GET/SET_MSR and
userspace will learn about the index automatically via
KVM_GET_MSR_INDEX_LIST.
Paolo
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