Re: [PATCH] kvm/vmx: EPTP switching test

From: =?UTF-8?q?Radim=20Kr=C4=8Dm=C3=A1=C5=99?=
Date: Mon Nov 16 2015 - 12:51:17 EST

2015-11-15 18:00+0200, Michael S. Tsirkin:
> This patch adds a new parameter: eptp_switching_test, which enables
> testing EPT switching on VMX if supported by hardware. All EPT entries
> are initialized to the same value so this adds no useful functionality
> by itself, but can be used to test VMFUNC performance, and serve as a
> basis for future features based on EPTP switching.
> Support for nested virt is not enabled.
> This was tested using the following code within guest:
> #define VMX_VMFUNC ".byte 0x0f,0x01,0xd4"
> static void vmfunc(unsigned int nr, unsigned int ept)
> {
> asm volatile(VMX_VMFUNC
> :
> : "a"(nr), "c"(ept)
> : "memory");
> }
> VMFUNC instruction cost was measured at ~122 cycles.
> (Note: recent versions of gnu toolchain support
> the vmfunc instruction - removing the need for writing
> the bytecode manually).
> Signed-off-by: Michael S. Tsirkin <mst@xxxxxxxxxx>
> ---
> I think I'd like to put this upstream so future eptp switching work can
> be implemented on top. Comments?

I'd wait for the future. Patch is already on the list so people
interested in benchmarking VMFUNC can quickly compile a kernel and
developers will need to overwrite the code anyway.

(And I think that eptp switching is expected to be used in conjuction
with #VE, so it'd then make sense to implement a nop for it as well.)

> diff --git a/arch/x86/include/asm/vmx.h b/arch/x86/include/asm/vmx.h
> @@ -3011,6 +3035,7 @@ static __init int setup_vmcs_config(struct vmcs_config *vmcs_conf)

The VMFUNC vmexit should be handled to prevent guests from triggering a
WARN_ON on the host. (VMFUNC did just #UD before this patch.)

After that, it's ok for KVM.
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