linux-kernel - Re: [PATCH] kvm/vmx: EPTP switching test

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Message-ID: <20151116175105.GA17144@potion.brq.redhat.com>
Date:	Mon, 16 Nov 2015 18:51:06 +0100
From:	=?UTF-8?q?Radim=20Kr=C4=8Dm=C3=A1=C5=99?= <rkrcmar@...hat.com>
To:	"Michael S. Tsirkin" <mst@...hat.com>
Cc:	linux-kernel@...r.kernel.org, Paolo Bonzini <pbonzini@...hat.com>,
	Wanpeng Li <wanpeng.li@...ux.intel.com>,
	Andy Lutomirski <luto@...nel.org>,
	Xiao Guangrong <guangrong.xiao@...ux.intel.com>,
	Kai Huang <kai.huang@...ux.intel.com>,
	=?UTF-8?q?Mihai=20Don=C8=9Bu?= 
	<mdontu@...defender.com>, kvm@...r.kernel.org
Subject: Re: [PATCH] kvm/vmx: EPTP switching test

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@...hat.com>
> ---
> 
> 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)
>  			SECONDARY_EXEC_PAUSE_LOOP_EXITING |
>  			SECONDARY_EXEC_RDTSCP |
>  			SECONDARY_EXEC_ENABLE_INVPCID |
> +			SECONDARY_EXEC_ENABLE_VM_FUNCTIONS |

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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