Re: [PATCH 0/4] KVM: VMX: Preemptivly optimize VMX instrs

[Paolo Bonzini <pbonzini@redhat.com>]

On 19/07/19 19:25, Sean Christopherson wrote:
> An in-flight patch[1] to make __kvm_handle_fault_on_reboot() play nice
> with objtool will add a JMP 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.
> 
> 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.
> 
> [1] https://lkml.kernel.org/r/64a9b64d127e87b6920a97afde8e96ea76f6524e.1563413318.git.jpoimboe@redhat.com
> 
> Sean Christopherson (4):
>   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()
> 
>  arch/x86/include/asm/kvm_host.h |  6 +--
>  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, 102 insertions(+), 43 deletions(-)
> 

Very nice - series

Acked-by: Paolo Bonzini <pbonzini@redhat.com>

Paolo