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: <CABgObfbwFPDiRbmVMtQZ9HipiT=4zXRqrE1fd7d44EeHt8b7=A@mail.gmail.com>
Date: Tue, 3 Sep 2024 18:49:50 +0200
From: Paolo Bonzini <pbonzini@...hat.com>
To: Sean Christopherson <seanjc@...gle.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 Tue, Sep 3, 2024 at 5:09 PM Sean Christopherson <seanjc@...gle.com> wrote:
> 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.

The other way round? i.e. treat EXIT_FASTPATH_REENTER_GUEST as
EXIT_FASTPATH_EXIT_HANDLED to go through event 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().

No, I wasn't worried about performance. Probably I misread

                if (likely(exit_fastpath != EXIT_FASTPATH_REENTER_GUEST))
                        break;

as something like

                if (likely(exit_fastpath == EXIT_FASTPATH_REENTER_GUEST))
                        continue;

EXIT_FASTPATH_REENTER_GUEST is exactly what's needed here.

Paolo