Re: [RFC 0/5] Making KVM_GET_ONE_REG/KVM_SET_ONE_REG generic.

[Avi Kivity <avi@redhat.com>]

On 09/05/2012 09:48 AM, Rusty Russell wrote:
> Peter Maydell <peter.maydell@linaro.org> writes:
>> On 1 September 2012 13:28, Rusty Russell <rusty@rustcorp.com.au> wrote:
>>> Rusty Russell (8):
>>>       KVM: ARM: Fix walk_msrs()
>>>       KVM: Move KVM_SET_ONE_REG/KVM_GET_ONE_REG to generic code.
>>>       KVM: Add KVM_REG_SIZE() helper.
>>>       KVM: ARM: use KVM_SET_ONE_REG/KVM_GET_ONE_REG.
>>>       KVM: Add KVM_VCPU_GET_REG_LIST.
>>>       KVM: ARM: Use KVM_VCPU_GET_REG_LIST.
>>>       KVM: ARM: Access all registers via KVM_GET_ONE_REG/KVM_SET_ONE_REG.
>>>       KVM ARM: Update api.txt
>>
>> So I was thinking about this, and I remembered that the SET_ONE_REG/
>> GET_ONE_REG API has userspace pass a pointer to the variable the
>> kernel should read/write (unlike the _MSR x86 ioctls, where the
>> actual data value is sent back and forth in the struct). Further,
>> the kernel only writes a data value of the size of the register
>> (rather than always reading/writing a uint64_t).
>>
>> This is a problem because it means userspace needs to know the
>> size of each register, and the kernel doesn't provide any way
>> to determine the size. This defeats the idea that userspace should
>> be able to migrate kernel register state without having to know
>> the semantics of all the registers involved.
> 
> It's there.  There are bits in the id which indicate the size:
> 
> #define KVM_REG_SIZE_SHIFT	52
> #define KVM_REG_SIZE_MASK	0x00f0000000000000ULL
> #define KVM_REG_SIZE_U8		0x0000000000000000ULL
> #define KVM_REG_SIZE_U16	0x0010000000000000ULL
> #define KVM_REG_SIZE_U32	0x0020000000000000ULL
> #define KVM_REG_SIZE_U64	0x0030000000000000ULL
> #define KVM_REG_SIZE_U128	0x0040000000000000ULL
> #define KVM_REG_SIZE_U256	0x0050000000000000ULL
> #define KVM_REG_SIZE_U512	0x0060000000000000ULL
> #define KVM_REG_SIZE_U1024	0x0070000000000000ULL
> 

Assumes power-of-two registers.  On x86 IDTR is 10 bytes long (2 byte
limit, 8 byte address).  We could split it into two registers, or add
padding, but it's unnatural.


-- 
error compiling committee.c: too many arguments to function