Re: [PATCH 15/17] KVM: retpolines: x86: eliminate retpoline from vmx.c exit handlers

Andrea Arcangeli
2019-09-23T21:08:42Z

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) {
case EXIT_REASON_VMCLEAR:
if (nested)
return handle_vmclear(vcpu);
else
return handle_vmx_instruction(vcpu);
case EXIT_REASON_VMCLEAR:
if (nested)
[..]

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

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

Thanks,
Andrea