From: "Nakajima, Jun" <jun.nakajima@intel.com>
To: Thomas Gleixner <tglx@linutronix.de>
Cc: "Liu, Jing2" <jing2.liu@intel.com>,
Paolo Bonzini <pbonzini@redhat.com>,
LKML <linux-kernel@vger.kernel.org>,
"x86@kernel.org" <x86@kernel.org>,
"kvm@vger.kernel.org" <kvm@vger.kernel.org>,
Dave Hansen <dave.hansen@linux.intel.com>,
Arjan van de Ven <arjan@linux.intel.com>,
Jing Liu <jing2.liu@linux.intel.com>,
"seanjc@google.com" <seanjc@google.com>,
"Cooper, Andrew" <andrew.cooper3@citrix.com>,
"Bae, Chang Seok" <chang.seok.bae@intel.com>
Subject: Re: Thoughts of AMX KVM support based on latest kernel
Date: Wed, 17 Nov 2021 04:52:35 +0000 [thread overview]
Message-ID: <16BF8BE6-B7B1-4F3E-B972-9D82CD2F23C8@intel.com> (raw)
In-Reply-To: <878rxn6h6t.ffs@tglx>
>
> On Nov 16, 2021, at 11:20 AM, Thomas Gleixner <tglx@linutronix.de> wrote:
>
> Jing,
>
> On Wed, Nov 10 2021 at 13:01, Liu, Jing2 wrote:
>
> more thoughts.
>
>> Once we start passthrough the XFD MSR, we need to save/restore
>> them at VM exit/entry time. If we immediately resume the guest
>> without enabling interrupts/preemptions (exit fast-path), we have no
>> issues. We don't need to save the MSR.
>
> Correct.
>
>> The question is how the host XFD MSR is restored while control is in
>> KVM.
>>
>> The XSAVE(S) instruction saves the (guest) state component[x] as 0 or
>> doesn't save when XFD[x] != 0. Accordingly, XRSTOR(S) cannot restore
>> that (guest state). And it is possible that XFD != 0 and the guest is using
>> extended feature at VM exit;
>
> You mean on creative guests which just keep AMX state alive and set
> XFD[AMX] = 1 to later restore it to XFD[AMX] = 0?
Typically a (usual) guest saves the AMX state for the previous process and sets XFD[AMX] = 1 for the next at context switch time, and a VM exit can happen anytime, e.g. right after XFD[AMX] = 1.
But this case is okay because the state is already saved by the guest.
If a (creative) guest wants to set XFD[AMX] = 1 for fun while keeping AMX state alive without saving the AXM state, it may lose the state after VM exit/entry. I think the right thing to do is to avoid such programming in the first place. Let me find out if we can add such notes in the programming references.
---
Jun
next prev parent reply other threads:[~2021-11-17 4:52 UTC|newest]
Thread overview: 23+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-11-10 13:01 Thoughts of AMX KVM support based on latest kernel Liu, Jing2
2021-11-16 12:18 ` Thomas Gleixner
2021-11-16 16:05 ` Sean Christopherson
2021-11-16 18:55 ` Thomas Gleixner
2021-11-16 19:49 ` Paolo Bonzini
2021-11-16 20:14 ` Sean Christopherson
2021-11-16 20:36 ` Thomas Gleixner
2021-11-16 22:11 ` Nakajima, Jun
2021-11-17 7:39 ` Paolo Bonzini
2021-11-16 20:12 ` Sean Christopherson
2021-11-16 23:35 ` Thomas Gleixner
2021-11-16 19:20 ` Thomas Gleixner
2021-11-17 4:52 ` Nakajima, Jun [this message]
2021-11-17 7:31 ` Paolo Bonzini
2021-11-17 10:15 ` Tian, Kevin
2021-11-17 12:24 ` Thomas Gleixner
2021-11-17 12:53 ` Paolo Bonzini
2021-11-18 23:17 ` Nakajima, Jun
2021-11-19 10:13 ` Thomas Gleixner
2021-11-19 15:41 ` Nakajima, Jun
2021-12-08 0:50 ` Rewording of Setting IA32_XFD[18] (Re: Thoughts of AMX KVM support based on latest kernel) Nakajima, Jun
2021-12-08 13:57 ` Paolo Bonzini
2021-12-10 21:53 ` Thomas Gleixner
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=16BF8BE6-B7B1-4F3E-B972-9D82CD2F23C8@intel.com \
--to=jun.nakajima@intel.com \
--cc=andrew.cooper3@citrix.com \
--cc=arjan@linux.intel.com \
--cc=chang.seok.bae@intel.com \
--cc=dave.hansen@linux.intel.com \
--cc=jing2.liu@intel.com \
--cc=jing2.liu@linux.intel.com \
--cc=kvm@vger.kernel.org \
--cc=linux-kernel@vger.kernel.org \
--cc=pbonzini@redhat.com \
--cc=seanjc@google.com \
--cc=tglx@linutronix.de \
--cc=x86@kernel.org \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).