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From: "Tian, Kevin" <kevin.tian@intel.com>
To: "Zhong, Yang" <yang.zhong@intel.com>,
	"x86@kernel.org" <x86@kernel.org>,
	"kvm@vger.kernel.org" <kvm@vger.kernel.org>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	"linux-doc@vger.kernel.org" <linux-doc@vger.kernel.org>,
	"linux-kselftest@vger.kernel.org"
	<linux-kselftest@vger.kernel.org>,
	"tglx@linutronix.de" <tglx@linutronix.de>,
	"mingo@redhat.com" <mingo@redhat.com>,
	"bp@alien8.de" <bp@alien8.de>,
	"dave.hansen@linux.intel.com" <dave.hansen@linux.intel.com>,
	"pbonzini@redhat.com" <pbonzini@redhat.com>,
	"corbet@lwn.net" <corbet@lwn.net>,
	"shuah@kernel.org" <shuah@kernel.org>
Cc: "Christopherson,, Sean" <seanjc@google.com>,
	"Nakajima, Jun" <jun.nakajima@intel.com>,
	"jing2.liu@linux.intel.com" <jing2.liu@linux.intel.com>,
	"Liu, Jing2" <jing2.liu@intel.com>,
	"Zeng, Guang" <guang.zeng@intel.com>,
	"Wang, Wei W" <wei.w.wang@intel.com>
Subject: RE: [PATCH v4 00/21] AMX Support in KVM
Date: Thu, 30 Dec 2021 08:46:42 +0000	[thread overview]
Message-ID: <BN9PR11MB527605F94C336B49584328278C459@BN9PR11MB5276.namprd11.prod.outlook.com> (raw)
In-Reply-To: <20211229131328.12283-1-yang.zhong@intel.com>

> From: Zhong, Yang <yang.zhong@intel.com>
> Sent: Wednesday, December 29, 2021 9:13 PM
> 
> 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.
> 
>   - 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.

Another option is to move xfd sync into vmx_vcpu_run(), given that
disabling xfd interception is vmx specific code thus it makes some
sense to also handle related side-effect in vmx.c. If this can be agreed
then yes there is no need of an extra flag and just checking msr_bitmap
is sufficient (and more accurate).

Paolo, how is your opinion?

> 
>   - 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 just pointed out some potential issues in current logic, about handling
#NM raised in L2 guest (could happen just due to L1 interception).

It's being discussed here:

	https://lore.kernel.org/all/YcyaN7V4wwGI7wDV@google.com/

Thanks
Kevin

  parent reply	other threads:[~2021-12-30  8:46 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 ` Tian, Kevin [this message]
2022-01-04 18:36 ` [PATCH v4 00/21] AMX Support in KVM Paolo Bonzini
2022-01-04 18:54   ` Sean Christopherson
2022-01-05  0:10     ` Tian, Kevin

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