From: "Wang, Wei W" <wei.w.wang@intel.com>
To: Sean Christopherson <seanjc@google.com>,
"Liu, Jing2" <jing2.liu@intel.com>
Cc: "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>,
"Nakajima, Jun" <jun.nakajima@intel.com>,
"Tian, Kevin" <kevin.tian@intel.com>,
"jing2.liu@linux.intel.com" <jing2.liu@linux.intel.com>,
"Zeng, Guang" <guang.zeng@intel.com>,
"Zhong, Yang" <yang.zhong@intel.com>
Subject: RE: [PATCH v3 19/22] kvm: x86: Get/set expanded xstate buffer
Date: Wed, 29 Dec 2021 02:57:58 +0000 [thread overview]
Message-ID: <96d1266eeeb8434f9e23f9a051e252e6@intel.com> (raw)
In-Reply-To: <YcuuCMCQryzUFoAZ@google.com>
On Wednesday, December 29, 2021 8:39 AM, Sean Christopherson wrote:
> To: Liu, Jing2 <jing2.liu@intel.com>
> Cc: 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;
> pbonzini@redhat.com; corbet@lwn.net; shuah@kernel.org; Nakajima, Jun
> <jun.nakajima@intel.com>; Tian, Kevin <kevin.tian@intel.com>;
> jing2.liu@linux.intel.com; Zeng, Guang <guang.zeng@intel.com>; Wang, Wei
> W <wei.w.wang@intel.com>; Zhong, Yang <yang.zhong@intel.com>
> Subject: Re: [PATCH v3 19/22] kvm: x86: Get/set expanded xstate buffer
>
> Shortlog needs to have a verb somewhere.
>
> On Wed, Dec 22, 2021, Jing Liu wrote:
> > From: Guang Zeng <guang.zeng@intel.com>
> >
> > When AMX is enabled it requires a larger xstate buffer than the legacy
> > hardcoded 4KB one. Exising kvm ioctls
>
> Existing
>
> > (KVM_[G|S]ET_XSAVE under KVM_CAP_XSAVE) are not suitable for this
> > purpose.
>
> ...
>
> > Reuse KVM_SET_XSAVE for both old/new formats by reimplementing it to
> > do properly-sized memdup_user() based on the guest fpu container.
>
> I'm confused, the first sentence says KVM_SET_XSAVE isn't suitable, the
> second says it can be reused with minimal effort.
Probably "doesn't support" sounds better than "isn't suitable" above. But plan to reword a bit:
With KVM_CAP_XSAVE, userspace uses a hardcoded 4KB buffer to get/set xstate data from/to
KVM. This doesn't work when dynamic features (e.g. AMX) are used by the guest, as KVM uses
a full expanded xstate buffer for the guest fpu emulation, which is larger than 4KB.
Add KVM_CAP_XSAVE2, and userspace gets the required xstate buffer size from KVM via
KVM_CHECK_EXTENSION(KVM_CAP_XSAVE2). KVM_SET_XSAVE is extended with the support to
work with larger xstate data size passed from userspace. KVM_GET_XSAVE2 is preferred to
extending KVM_GET_XSAVE to work with large buffer size for backward-compatible considerations.
(Link: https://lkml.org/lkml/2021/12/15/510)
Also, update the api doc with the new KVM_GET_XSAVE2 ioctl.
>
> > Also, update the api doc with the new KVM_GET_XSAVE2 ioctl.
>
> ...
>
> > @@ -5367,7 +5382,9 @@ long kvm_arch_vcpu_ioctl(struct file *filp,
> > break;
> > }
> > case KVM_SET_XSAVE: {
> > - u.xsave = memdup_user(argp, sizeof(*u.xsave));
> > + int size = vcpu->arch.guest_fpu.uabi_size;
>
> IIUC, reusing KVM_SET_XSAVE works by requiring that userspace use
> KVM_GET_XSAVE2 if userspace has expanded the guest FPU size by exposing
> relevant features to the guest via guest CPUID. If so, then that needs to be
> enforced in KVM_GET_XSAVE, otherwise userspace will get subtle corruption
> by invoking the wrong ioctl, e.g.
>
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c index
> 2c9606380bca..5d2acbd52df5 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -5386,6 +5386,10 @@ long kvm_arch_vcpu_ioctl(struct file *filp,
> break;
> }
> case KVM_GET_XSAVE: {
> + r -EINVAL;
> + if (vcpu->arch.guest_fpu.uabi_size > sizeof(struct
> kvm_xsave))
> + break;
> +
Looks good to me.
Thanks,
Wei
next prev parent reply other threads:[~2021-12-29 2:58 UTC|newest]
Thread overview: 47+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-12-22 12:40 [PATCH v3 00/22] AMX Support in KVM Jing Liu
2021-12-22 12:40 ` [PATCH v3 01/22] x86/fpu: Extend fpu_xstate_prctl() with guest permissions Jing Liu
2021-12-22 12:40 ` [PATCH v3 02/22] x86/fpu: Prepare guest FPU for dynamically enabled FPU features Jing Liu
2021-12-22 12:40 ` [PATCH v3 03/22] kvm: x86: Fix xstate_required_size() to follow XSTATE alignment rule Jing Liu
2021-12-22 12:40 ` [PATCH v3 04/22] kvm: x86: Exclude unpermitted xfeatures at KVM_GET_SUPPORTED_CPUID Jing Liu
2021-12-22 12:40 ` [PATCH v3 05/22] kvm: x86: Check permitted dynamic xfeatures at KVM_SET_CPUID2 Jing Liu
2021-12-28 23:38 ` Sean Christopherson
2021-12-29 2:18 ` Tian, Kevin
2021-12-22 12:40 ` [PATCH v3 06/22] x86/fpu: Make XFD initialization in __fpstate_reset() a function argument Jing Liu
2021-12-22 12:40 ` [PATCH v3 07/22] x86/fpu: Add guest support to xfd_enable_feature() Jing Liu
2021-12-22 12:40 ` [PATCH v3 08/22] x86/fpu: Provide fpu_update_guest_perm_features() for guest Jing Liu
2021-12-22 12:40 ` [PATCH v3 09/22] kvm: x86: Enable dynamic XSAVE features at KVM_SET_CPUID2 Jing Liu
2021-12-28 23:54 ` Sean Christopherson
2021-12-29 2:23 ` Tian, Kevin
2021-12-22 12:40 ` [PATCH v3 10/22] x86/fpu: Provide fpu_update_guest_xfd() for IA32_XFD emulation Jing Liu
2021-12-22 12:40 ` [PATCH v3 11/22] kvm: x86: Add emulation for IA32_XFD Jing Liu
2021-12-22 12:40 ` [PATCH v3 12/22] x86/fpu: Prepare xfd_err in struct fpu_guest Jing Liu
2021-12-22 12:40 ` [PATCH v3 13/22] kvm: x86: Intercept #NM for saving IA32_XFD_ERR Jing Liu
2021-12-29 0:09 ` Sean Christopherson
2021-12-29 2:52 ` Tian, Kevin
2021-12-29 17:37 ` Sean Christopherson
2021-12-29 6:50 ` Tian, Kevin
2021-12-29 8:13 ` Tian, Kevin
2021-12-22 12:40 ` [PATCH v3 14/22] kvm: x86: Emulate IA32_XFD_ERR for guest Jing Liu
2021-12-22 12:40 ` [PATCH v3 15/22] kvm: x86: Disable RDMSR interception of IA32_XFD_ERR Jing Liu
2021-12-22 12:40 ` [PATCH v3 16/22] kvm: x86: Add XCR0 support for Intel AMX Jing Liu
2021-12-29 0:21 ` Sean Christopherson
2021-12-29 3:01 ` Tian, Kevin
2021-12-22 12:40 ` [PATCH v3 17/22] kvm: x86: Add CPUID " Jing Liu
2021-12-22 12:40 ` [PATCH v3 18/22] x86/fpu: Add uabi_size to guest_fpu Jing Liu
2021-12-22 12:40 ` [PATCH v3 19/22] kvm: x86: Get/set expanded xstate buffer Jing Liu
2021-12-29 0:38 ` Sean Christopherson
2021-12-29 2:57 ` Wang, Wei W [this message]
2021-12-29 6:36 ` Tian, Kevin
2021-12-22 12:40 ` [PATCH v3 20/22] kvm: selftests: Add support for KVM_CAP_XSAVE2 Jing Liu
2021-12-22 12:40 ` [PATCH v3 21/22] x86/fpu: Provide fpu_sync_guest_vmexit_xfd_state() Jing Liu
2021-12-22 12:40 ` [PATCH v3 22/22] kvm: x86: Disable interception for IA32_XFD on demand Jing Liu
2021-12-29 1:04 ` Sean Christopherson
2021-12-29 3:35 ` Tian, Kevin
2021-12-29 7:16 ` Tian, Kevin
2021-12-29 17:26 ` Sean Christopherson
2021-12-30 1:28 ` Tian, Kevin
2021-12-30 7:04 ` Tian, Kevin
2021-12-31 9:42 ` Tian, Kevin
2021-12-29 7:37 ` Tian, Kevin
2022-01-04 18:32 ` Paolo Bonzini
2022-01-04 18:58 ` Sean Christopherson
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