linux-kernel - Re: [PATCH 15/19] kvm: x86: Save and restore guest XFD

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Date:   Sun, 12 Dec 2021 10:10:19 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     "Tian, Kevin" <kevin.tian@...el.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        "Zhong, Yang" <yang.zhong@...el.com>,
        "x86@...nel.org" <x86@...nel.org>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "bp@...en8.de" <bp@...en8.de>,
        "dave.hansen@...ux.intel.com" <dave.hansen@...ux.intel.com>
Cc:     "Christopherson,, Sean" <seanjc@...gle.com>,
        "Nakajima, Jun" <jun.nakajima@...el.com>,
        "jing2.liu@...ux.intel.com" <jing2.liu@...ux.intel.com>,
        "Liu, Jing2" <jing2.liu@...el.com>
Subject: Re: [PATCH 15/19] kvm: x86: Save and restore guest XFD_ERR properly

On 12/12/21 02:50, Tian, Kevin wrote:
>>
>> If there are other XFD controlled facilities in the future, then it will
>> be NR_USED_XFD_CONTROLLED_FACILITIES * 2 VMEXITs per thread which
>> uses
>> them. Not the end of the world either.
>>
>> Looking at the targeted application space it's pretty unlikely that
>> tasks which utilize AMX are going to be so short lived that the overhead
>> of these VMEXITs really matters.
>>
>> This of course can be revisited when there is a sane use case, but
>> optimizing for it prematurely does not buy us anything else than
>> pointless complexity.
> It may affect guest which still uses CR0.TS to do lazy save. But likely
> modern OSes all move to eager save approach so always trapping #NM
> should be fine.

You also don't need to trap #NM if CPUID includes no dynamic bits, 
because then XFD will never be nonzero.

Paolo