On Tue, Mar 27, 2018 at 8:35 AM, Sean Christopherson
<sean.j.christopherson@xxxxxxxxx> wrote:
> On Tue, 2018-03-27 at 10:50 +0200, David Hildenbrand wrote:
>> On 27.03.2018 02:40, Jidong Xiao wrote:
>> >
>> > Hi,
>> >
>> > The SDM defines the opcode of the VMXON instruction as F3 0F C7 /6,
>> > yet in the kernel source code (arch/x86/include/asm/vmx.h) I saw:
>> >
>> > #define ASM_VMX_VMXON_RAX ".byte 0xf3, 0x0f, 0xc7, 0x30"
>> >
>> > So what is 0x30 used for? And why there is no "/6"?
>> The /6 actually refers to the ModR/M byte if I'm not mistaking.
>
> Yep.
>
> /digit — A digit between 0 and 7 indicates that the ModR/M byte of
> the instruction uses only the r/m (register or memory)
> operand. The reg field contains the digit that
> provides
> an extension to the instruction's opcode.
>
>> It is used to define how the address is passed to the VMXON instruction.
>> 0x30 refers to RAX here. So if you would want to pass the address e.g.
>> via RBX, you would need 0x33 if I'm not wrong.
>
> Yep again. 0x30 is hardcoding the ModR/M to Mod=00b, Reg=110b (this
> is the /6 digit) and R/M=000b, which is equivalent to "VMXON [RAX]";
> thus the _RAX at the end of the macro name.
>
> The mnemonic "VMXON m64" in the SDM states that VMXON must have a memory
> operand, i.e. Mod=11b will result in a #UD. The other memory Mods (01b
> and 10b) are legal for VMXON, but KVM undoubtedly added VMXON prior to
> widespread compiler support for VMX, e.g. asm volatile("vmxon...") wasn't
> an option. Hardcoding a specific ModR/M and stuffing the appropriate reg
> is a clean and easy method to hand encode an opcode with a /digit field.
>
> Section 3.1 in Vol. 2A of the current SDM has more details.
>
>> > I saw this 0x30 in several other macros as well, such as
>> > ASM_VMX_VMCLEAR_RAX and ASM_VMX_VMPTRLD_RAX.
>> >
>> > Where in the SDM, can I find some explanation to this 0x30? Thanks.
>> Look out for the ModR/M tables.
>>
Thanks Sean and David! It took me quite a while to understand why you
said here; but after referring the SDM (and other online documents
explaining the Intel instructions format) and reading your
explanation, finally I understand that piece of code used in KVM.
Thanks again, it's very helpful!
-Jidong
