linux-kernel - Re: [PATCH v2 0/5] KVM: VMX: Optimize VMX instrs error/fault handling

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Date:   Tue, 24 Sep 2019 14:03:37 -0700
From:   Sean Christopherson <sean.j.christopherson@...el.com>
To:     Paolo Bonzini <pbonzini@...hat.com>,
        Radim Krčmář <rkrcmar@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        x86@...nel.org, Josh Poimboeuf <jpoimboe@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>
Cc:     "H. Peter Anvin" <hpa@...or.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 0/5] KVM: VMX: Optimize VMX instrs error/fault handling

Paolo,

Any chance this can be picked up for 5.4?  Josh's kvm_spurious_fault()
changes went into 5.3, so this can be taken through the KVM tree, which
probably makes the most sense since the non-KVM change is a one line
deletion.

On Fri, Jul 19, 2019 at 01:41:05PM -0700, Sean Christopherson wrote:
> A recent commit reworked __kvm_handle_fault_on_reboot() to play nice with
> objtool.  An unfortunate side effect is that JMP is now inserted after
> most VMX instructions so that the reboot macro can use an actual CALL to
> kvm_spurious_fault() instead of a funky PUSH+JMP facsimile in .fixup.
> 
> Rework the low level VMX instruction helpers to handle unexpected faults
> manually instead of relying on the "fault on reboot" macro.  By using
> asm-goto, most helpers can branch directly to an in-function call to
> kvm_spurious_fault(), which can then be optimized by compilers to reside
> out-of-line at the end of the function instead of inline as done by
> "fault on reboot".
> 
> The net impact relative to the current code base is more or less a nop
> when building with a compiler that supports __GCC_ASM_FLAG_OUTPUTS__.
> A bunch of code that was previously in .fixup gets moved into the slow
> paths of functions, but the fast paths are more basically unchanged.
> 
> Without __GCC_ASM_FLAG_OUTPUTS__, manually coding the Jcc is a net
> positive as CC_SET() without compiler support almost always generates a
> SETcc+CMP+Jcc sequence, which is now replaced with a single Jcc.
> 
> A small bonus is that the Jcc instrs are hinted to predict that the VMX
> instr will be successful.
> 
> v2:
>   - Rebased to x86/master, commit eceffd88ca20 ("Merge branch 'x86/urgent'")
>   - Reworded changelogs to reference the commit instead lkml link for
>     the recent changes to __kvm_handle_fault_on_reboot().
>   - Added Paolo's acks for patch 1-4
>   - Added patch 5 to do more cleanup, which was made possible by rebasing
>     on top of the __kvm_handle_fault_on_reboot() changes.
>   
> Sean Christopherson (5):
>   objtool: KVM: x86: Check kvm_rebooting in kvm_spurious_fault()
>   KVM: VMX: Optimize VMX instruction error and fault handling
>   KVM: VMX: Add error handling to VMREAD helper
>   KVM: x86: Drop ____kvm_handle_fault_on_reboot()
>   KVM: x86: Don't check kvm_rebooting in __kvm_handle_fault_on_reboot()
> 
>  arch/x86/include/asm/kvm_host.h | 16 ++----
>  arch/x86/kvm/vmx/ops.h          | 93 ++++++++++++++++++++-------------
>  arch/x86/kvm/vmx/vmx.c          | 42 +++++++++++++++
>  arch/x86/kvm/x86.c              |  3 +-
>  tools/objtool/check.c           |  1 -
>  5 files changed, 104 insertions(+), 51 deletions(-)
> 
> -- 
> 2.22.0
>