linux-kernel - Re: [PATCH 1/5] KVM: x86: Re-enter guest if WRMSR(X2APIC

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: <ZtcmtFlX83g7C8Vd@google.com>
Date: Tue, 3 Sep 2024 08:09:40 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Paolo Bonzini <pbonzini@...hat.com>
Cc: kvm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/5] KVM: x86: Re-enter guest if WRMSR(X2APIC_ICR)
 fastpath is successful

On Mon, Sep 02, 2024, Paolo Bonzini wrote:
> On Fri, Aug 2, 2024 at 9:51 PM Sean Christopherson <seanjc@...gle.com> wrote:
> > Re-enter the guest in the fastpath if WRMSR emulation for x2APIC's ICR is
> > successful, as no additional work is needed, i.e. there is no code unique
> > for WRMSR exits between the fastpath and the "!= EXIT_FASTPATH_NONE" check
> > in __vmx_handle_exit().
> 
> What about if you send an IPI to yourself?  Doesn't that return true
> for kvm_vcpu_exit_request() if posted interrupts are disabled?

Yes, but that doesn't have anything to do with WRMSR itself, as KVM needs to morph
EXIT_FASTPATH_EXIT_HANDLED => EXIT_FASTPATH_REENTER_GUEST if there's a pending
event that needs requires injection.

Given that kvm_x86_ops.sync_pir_to_irr is likely NULL if virtual interrupt delivery
is enabled, the overhead of the trying to re-enter the guest it essentially a few
cycles, e.g. check vcpu->mode and kvm_request_pending().