Peter Maydell <[email protected]> writes:
> On 1 September 2012 13:28, Rusty Russell <[email protected]> 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
And my patches added a helper:
#define KVM_REG_SIZE(id) \
(1U << (((id) & KVM_REG_SIZE_MASK) >> KVM_REG_SIZE_SHIFT))
> I could live with "always read/write 64 bits". I definitely don't
> want to have to deal with matching up register widths to accesses
> in userspace, please.
I changed my mind about the old scheme when I realized we have to deal
with 128-bit FPU registers.
Cheers,
Rusty.
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