On 09/05/2012 09:48 AM, Rusty Russell wrote:

Peter Maydell [off-list ref] writes:

quoted

On 1 September 2012 13:28, Rusty Russell [off-list ref] wrote:

quoted

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