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 for Android: free password hash cracker in your pocket

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

Message-ID: <Z6ucl7U79RuBsYJt@google.com>
Date: Tue, 11 Feb 2025 10:53:11 -0800
From: Sean Christopherson <seanjc@...gle.com>
To: 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, 
	vkuznets@...hat.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

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