From: Paolo Bonzini <pbonzini@redhat.com>
To: Sean Christopherson <seanjc@google.com>
Cc: Jing Liu <jing2.liu@linux.intel.com>,
kvm@vger.kernel.org, linux-kernel@vger.kernel.org,
jing2.liu@intel.com
Subject: Re: [PATCH RFC 3/7] kvm: x86: XSAVE state and XFD MSRs context switch
Date: Mon, 8 Feb 2021 18:45:35 +0100 [thread overview]
Message-ID: <77b27707-721a-5c6a-c00d-e1768da55c64@redhat.com> (raw)
In-Reply-To: <YCF1d0F0AqPazYqC@google.com>
On 08/02/21 18:31, Sean Christopherson wrote:
> On Mon, Feb 08, 2021, Paolo Bonzini wrote:
>> On 07/02/21 16:42, Jing Liu wrote:
>>> In KVM, "guest_fpu" serves for any guest task working on this vcpu
>>> during vmexit and vmenter. We provide a pre-allocated guest_fpu space
>>> and entire "guest_fpu.state_mask" to avoid each dynamic features
>>> detection on each vcpu task. Meanwhile, to ensure correctly
>>> xsaves/xrstors guest state, set IA32_XFD as zero during vmexit and
>>> vmenter.
>>
>> Most guests will not need the whole xstate feature set. So perhaps you
>> could set XFD to the host value | the guest value, trap #NM if the host XFD
>> is zero, and possibly reflect the exception to the guest's XFD and XFD_ERR.
>>
>> In addition, loading the guest XFD MSRs should use the MSR autoload feature
>> (add_atomic_switch_msr).
>
> Why do you say that? I would strongly prefer to use the load lists only if they
> are absolutely necessary. I don't think that's the case here, as I can't
> imagine accessing FPU state in NMI context is allowed, at least not without a
> big pile of save/restore code.
I was thinking more of the added vmentry/vmexit overhead due to
xfd_guest_enter xfd_guest_exit.
That said, the case where we saw MSR autoload as faster involved EFER,
and we decided that it was due to TLB flushes (commit f6577a5fa15d,
"x86, kvm, vmx: Always use LOAD_IA32_EFER if available", 2014-11-12).
Do you know if RDMSR/WRMSR is always slower than MSR autoload?
Paolo
next prev parent reply other threads:[~2021-02-08 17:49 UTC|newest]
Thread overview: 39+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-02-07 15:42 [PATCH RFC 0/7] Introduce support for guest AMX feature Jing Liu
2021-02-07 15:42 ` [PATCH RFC 1/7] kvm: x86: Expose XFD CPUID to guest Jing Liu
2021-05-24 21:34 ` Sean Christopherson
2021-06-07 3:27 ` Liu, Jing2
2021-02-07 15:42 ` [PATCH RFC 2/7] kvm: x86: Introduce XFD MSRs as passthrough " Jing Liu
2021-05-24 21:43 ` Sean Christopherson
2021-05-24 21:57 ` Jim Mattson
2021-06-02 3:12 ` Liu, Jing2
2021-06-23 17:50 ` Dave Hansen
2021-06-28 2:00 ` Liu, Jing2
2021-06-29 17:58 ` Dave Hansen
2021-07-06 7:33 ` Liu, Jing2
2021-02-07 15:42 ` [PATCH RFC 3/7] kvm: x86: XSAVE state and XFD MSRs context switch Jing Liu
2021-02-07 11:49 ` Borislav Petkov
2021-02-08 3:35 ` Liu, Jing2
2021-02-08 10:25 ` Paolo Bonzini
2021-02-08 17:31 ` Sean Christopherson
2021-02-08 17:45 ` Paolo Bonzini [this message]
2021-02-08 18:04 ` Sean Christopherson
2021-02-08 18:12 ` Paolo Bonzini
2021-02-08 18:55 ` Konrad Rzeszutek Wilk
2021-02-22 8:51 ` Liu, Jing2
2021-02-22 8:36 ` Liu, Jing2
2021-02-07 15:42 ` [PATCH RFC 4/7] kvm: x86: Add new ioctls for XSAVE extension Jing Liu
2021-05-24 21:50 ` Sean Christopherson
2021-05-26 6:09 ` Liu, Jing2
2021-05-26 14:43 ` Sean Christopherson
2021-06-01 10:24 ` Liu, Jing2
2021-06-07 5:23 ` Liu, Jing2
2021-05-24 22:06 ` Jim Mattson
2021-05-26 6:11 ` Liu, Jing2
2021-02-07 15:42 ` [PATCH RFC 5/7] kvm: x86: Revise CPUID.D.1.EBX for alignment rule Jing Liu
2021-05-24 21:28 ` Sean Christopherson
2021-06-03 4:45 ` Liu, Jing2
2021-02-07 15:42 ` [PATCH RFC 6/7] kvm: x86: Add AMX_TILE, AMX_INT8 and AMX_BF16 support Jing Liu
2021-02-07 15:42 ` [PATCH RFC 7/7] kvm: x86: AMX XCR0 support for guest Jing Liu
2021-05-24 21:53 ` Sean Christopherson
2021-05-26 7:54 ` Liu, Jing2
2021-05-26 14:54 ` Sean Christopherson
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