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

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On 1 September 2012 13:28, Rusty Russell <rusty@xxxxxxxxxxxxxxx> 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.

Possible solutions:
 * switch GET/SET_ONE_REG to just passing data, same as the MSR ioctls
 * switch GET/SET_ONE_REG to always writing 64 bits regardless of
   actual guest register width
 * make GET_REG_LIST return register width as well as index

Personally I would really prefer the MSR-style "pass the data".
Otherwise I'm going to end up constructing something like
 uint64_t actual_values[]
 struct kvm_one_reg regs[]

where regs[x].addr = &actual_values[x] for all x. Which seems
like unnecessary indirection really :-)

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.

thanks
-- PMM
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