[lkml]   [2019]   [Sep]   [23]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
Messages in this thread
SubjectRe: [PATCH 15/17] KVM: retpolines: x86: eliminate retpoline from vmx.c exit handlers
On Mon, Sep 23, 2019 at 05:08:38PM -0400, Andrea Arcangeli wrote:
> Hello,
> On Mon, Sep 23, 2019 at 01:23:49PM -0700, Sean Christopherson wrote:
> > The attached patch should do the trick.
> The two most attractive options to me remains what I already have
> implemented under #ifdef CONFIG_RETPOLINE with direct calls
> (optionally replacing the "if" with a small "switch" still under
> CONFIG_RETPOLINE if we give up the prioritization of the checks), or
> the replacement of kvm_vmx_exit_handlers with a switch() as suggested
> by Vitaly which would cleanup some code.
> The intermediate solution that makes "const" work, has the cons of
> forcing to parse EXIT_REASON_VMCLEAR and the other vmx exit reasons
> twice, first through a pointer to function (or another if or switch
> statement) then with a second switch() statement.
> If we'd use a single switch statement per Vitaly's suggestion, the "if
> nested" would better be more simply implemented as:
> switch (exit_reason) {
> if (nested)
> return handle_vmclear(vcpu);
> else
> return handle_vmx_instruction(vcpu);
> if (nested)
> [..]

Combing if/case statements would mitigate this to some degree, e.g.:

if (exit_reason >= EXIT_REASON_VMCALL &&
exit_reason <= EXIT_REASON_VMON)
return handle_vmx_instruction(vcpu);


return handle_vmx_instruction(vcpu);

> This also removes the compiler dependency to auto inline
> handle_vmclear in the added nested_vmx_handle_vmx_instruction extern
> call.

I like the idea of routing through handle_vmx_instruction() because it
allows the individual handlers to be wholly encapsulated in nested.c,
e.g. handle_vmclear() and company don't have to be exposed.

An extra CALL+RET isn't going to be noticeable, especially on modern
hardware as the high frequency VMWRITE/VMREAD fields should hit the
shadow VMCS.

> VMREAD/WRITE/RESUME are the most frequent vmexit in l0 while nested
> runs in l2.
> Thanks,
> Andrea

 \ /
  Last update: 2019-09-23 23:25    [W:0.114 / U:3.724 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site