linux-kernel - Re: [PATCH 1/2] KVM: x86: async_pf: remove support for KVM_ASYNC_PF_SEND

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]

Message-ID: <87frkcrab8.fsf@redhat.com>
Date: Mon, 17 Feb 2025 14:05:15 +0100
From: Vitaly Kuznetsov <vkuznets@...hat.com>
To: Sean Christopherson <seanjc@...gle.com>, Nikita Kalyazin
 <kalyazin@...zon.com>
Cc: pbonzini@...hat.com, corbet@....net, tglx@...utronix.de,
 mingo@...hat.com, bp@...en8.de, dave.hansen@...ux.intel.com,
 x86@...nel.org, hpa@...or.com, xiaoyao.li@...el.com, kvm@...r.kernel.org,
 linux-kernel@...r.kernel.org, linux-doc@...r.kernel.org,
 roypat@...zon.co.uk, xmarcalx@...zon.com
Subject: Re: [PATCH 1/2] KVM: x86: async_pf: remove support for
 KVM_ASYNC_PF_SEND_ALWAYS

Sean Christopherson <seanjc@...gle.com> writes:

> On Wed, Nov 27, 2024, Nikita Kalyazin wrote:
>> 3a7c8fafd1b42adea229fd204132f6a2fb3cd2d9 ("x86/kvm: Restrict
>> ASYNC_PF to user space") stopped setting KVM_ASYNC_PF_SEND_ALWAYS in
>> Linux guests.  While the flag can still be used by legacy guests, the
>> mechanism is best effort so KVM is not obliged to use it.
>
> What's the actual motivation to remove it from KVM?  I agreed KVM isn't required
> to honor KVM_ASYNC_PF_SEND_ALWAYS from a guest/host ABI perspective, but that
> doesn't mean that dropping a feature has no impact.  E.g. it's entirely possible
> removing this support could negatively affect a workload running on an old kernel.
>
> Looking back at the discussion[*] where Vitaly made this suggestion, I don't see
> anything that justifies dropping this code.  It costs KVM practically nothing to
> maintain this code.
>
> [*] https://lore.kernel.org/all/20241118130403.23184-1-kalyazin@amazon.com
>

How old is old? :-)

Linux stopped using KVM_ASYNC_PF_SEND_ALWAYS in v5.8: 

commit 3a7c8fafd1b42adea229fd204132f6a2fb3cd2d9
Author: Thomas Gleixner <tglx@...utronix.de>
Date:   Fri Apr 24 09:57:56 2020 +0200

    x86/kvm: Restrict ASYNC_PF to user space

and I was under the impression other OSes never used KVM asynchronous
page-fault in the first place (not sure about *BSDs though but certainly
not Windows). As Nikita's motivation for the patch was "to avoid the
overhead ... in case of kernel-originated faults" I suggested we start
by simplifyign the code to not care about 'send_user_only' at all. 

We can keep the code around, I guess, but with no plans to re-introduce
KVM_ASYNC_PF_SEND_ALWAYS usage to Linux I still believe it would be good
to set a deprecation date.

-- 
Vitaly