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From: Sean Christopherson <seanjc@google.com>
To: Paolo Bonzini <pbonzini@redhat.com>
Cc: Yang Zhong <yang.zhong@intel.com>,
	x86@kernel.org, kvm@vger.kernel.org,
	linux-kernel@vger.kernel.org, linux-doc@vger.kernel.org,
	linux-kselftest@vger.kernel.org, tglx@linutronix.de,
	mingo@redhat.com, bp@alien8.de, dave.hansen@linux.intel.com,
	corbet@lwn.net, shuah@kernel.org, jun.nakajima@intel.com,
	kevin.tian@intel.com, jing2.liu@linux.intel.com,
	jing2.liu@intel.com, guang.zeng@intel.com, wei.w.wang@intel.com
Subject: Re: [PATCH v4 00/21] AMX Support in KVM
Date: Tue, 4 Jan 2022 18:54:32 +0000	[thread overview]
Message-ID: <YdSX6NANtx0SXjfK@google.com> (raw)
In-Reply-To: <d43887b6-630c-446e-caee-dcbaa72f2466@redhat.com>

On Tue, Jan 04, 2022, Paolo Bonzini wrote:
> On 12/29/21 14:13, Yang Zhong wrote:
> > Highly appreciate for your review. This version mostly addressed the comments
> > from Sean. Most comments are adopted except three which are not closed and
> > need more discussions:
> > 
> >    - Move the entire xfd write emulation code to x86.c. Doing so requires
> >      introducing a new kvm_x86_ops callback to disable msr write bitmap.
> >      According to Paolo's earlier comment he prefers to handle it in vmx.c.
> 
> Yes, I do.

No objection, my comments were prior to seeing the patches that manipulated the
bitmap, e.g. in the earlier patches, having anything in vmx.c is unnecessary.
 
> >    - Directly check msr_bitmap in update_exception_bitmap() (for
> >      trapping #NM) and vcpu_enter_guest() (for syncing guest xfd after
> >      vm-exit) instead of introducing an extra flag in the last patch. However,
> >      doing so requires another new kvm_x86_ops callback for checking
> >      msr_bitmap since vcpu_enter_guest() is x86 common code. Having an
> >      extra flag sounds simpler here (at least for the initial AMX support).
> >      It does penalize nested guest with one xfd sync per exit, but it's not
> >      worse than a normal guest which initializes xfd but doesn't run
> >      AMX applications at all. Those could be improved afterwards.
> 
> The thing to do here would be to move
> MAX_POSSIBLE_PASSTHROUGH_MSRS/MAX_DIRECT_ACCESS_MSRS from VMX/SVM to core
> code.  For now we can keep the flag.
> 
> >    - Disable #NM trap for nested guest. This version still chooses to always
> >      trap #NM (regardless in L1 or L2) as long as xfd write interception is disabled.
> >      In reality #NM is rare if nested guest doesn't intend to run AMX applications
> >      and always-trap is safer than dynamic trap for the basic support in case
> >      of any oversight here.
> 
> Sean was justifying this with lack of support for nested AMX, but I'm not
> sure actually what is missing at all.  That is, an L1 hypervisor could
> expose AMX to L2, and then an L2->L0->L2 exit/reentry would have to trap
> #NM.  Otherwise it would miss an XFD_ERR update.

Ya, I was assuming there was something L0 needed to do to supported nested AMX,
but as Paolo pointed out there are no VMCS bits, so L0 just needs to correctly
handle #NM and MSR interceptions according to vmcs12.

  reply	other threads:[~2022-01-04 18:54 UTC|newest]

Thread overview: 38+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-12-29 13:13 [PATCH v4 00/21] AMX Support in KVM Yang Zhong
2021-12-29 13:13 ` [PATCH v4 01/21] x86/fpu: Extend fpu_xstate_prctl() with guest permissions Yang Zhong
2021-12-29 13:13 ` [PATCH v4 02/21] x86/fpu: Prepare guest FPU for dynamically enabled FPU features Yang Zhong
2021-12-29 13:13 ` [PATCH v4 03/21] kvm: x86: Fix xstate_required_size() to follow XSTATE alignment rule Yang Zhong
2022-01-04 19:54   ` Sean Christopherson
2021-12-29 13:13 ` [PATCH v4 04/21] kvm: x86: Exclude unpermitted xfeatures at KVM_GET_SUPPORTED_CPUID Yang Zhong
2021-12-29 13:13 ` [PATCH v4 05/21] x86/fpu: Make XFD initialization in __fpstate_reset() a function argument Yang Zhong
2021-12-29 13:13 ` [PATCH v4 06/21] x86/fpu: Add guest support to xfd_enable_feature() Yang Zhong
2021-12-29 13:13 ` [PATCH v4 07/21] x86/fpu: Provide fpu_update_guest_perm_features() for guest Yang Zhong
2021-12-29 13:13 ` [PATCH v4 08/21] kvm: x86: Check and enable permitted dynamic xfeatures at KVM_SET_CPUID2 Yang Zhong
2021-12-29 16:55   ` Sean Christopherson
2021-12-30  2:28     ` Tian, Kevin
2021-12-29 13:13 ` [PATCH v4 09/21] x86/fpu: Provide fpu_update_guest_xfd() for IA32_XFD emulation Yang Zhong
2021-12-29 13:13 ` [PATCH v4 10/21] kvm: x86: Add emulation for IA32_XFD Yang Zhong
2022-01-04 19:32   ` Sean Christopherson
2022-01-05  0:22     ` Tian, Kevin
2021-12-29 13:13 ` [PATCH v4 11/21] x86/fpu: Prepare xfd_err in struct fpu_guest Yang Zhong
2021-12-29 13:13 ` [PATCH v4 12/21] kvm: x86: Intercept #NM for saving IA32_XFD_ERR Yang Zhong
2022-01-04 20:01   ` Sean Christopherson
2022-01-05  0:27     ` Tian, Kevin
2021-12-29 13:13 ` [PATCH v4 13/21] kvm: x86: Emulate IA32_XFD_ERR for guest Yang Zhong
2021-12-29 13:13 ` [PATCH v4 14/21] kvm: x86: Disable RDMSR interception of IA32_XFD_ERR Yang Zhong
2022-01-04 19:34   ` Sean Christopherson
2022-01-05  0:23     ` Tian, Kevin
2021-12-29 13:13 ` [PATCH v4 15/21] kvm: x86: Add XCR0 support for Intel AMX Yang Zhong
2021-12-29 13:13 ` [PATCH v4 16/21] kvm: x86: Add CPUID " Yang Zhong
2021-12-29 13:13 ` [PATCH v4 17/21] x86/fpu: Add uabi_size to guest_fpu Yang Zhong
2021-12-29 13:13 ` [PATCH v4 18/21] kvm: x86: Add support for getting/setting expanded xstate buffer Yang Zhong
2022-01-04 19:46   ` Sean Christopherson
2022-01-04 20:45     ` Paolo Bonzini
2022-01-05  4:03       ` Tian, Kevin
2021-12-29 13:13 ` [PATCH v4 19/21] kvm: selftests: Add support for KVM_CAP_XSAVE2 Yang Zhong
2021-12-29 13:13 ` [PATCH v4 20/21] x86/fpu: Provide fpu_sync_guest_vmexit_xfd_state() Yang Zhong
2021-12-29 13:13 ` [PATCH v4 21/21] kvm: x86: Disable interception for IA32_XFD on demand Yang Zhong
2021-12-30  8:46 ` [PATCH v4 00/21] AMX Support in KVM Tian, Kevin
2022-01-04 18:36 ` Paolo Bonzini
2022-01-04 18:54   ` Sean Christopherson [this message]
2022-01-05  0:10     ` Tian, Kevin

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