Re: [PATCH] kvm/x86: reserve bit KVM_HINTS_PHYS_ADDRESS_SIZE_DATA_VALID

[Vitaly Kuznetsov]

Gerd Hoffmann <kraxel@xxxxxxxxxx> writes:

> On Thu, Sep 08, 2022 at 02:52:36PM +0000, Sean Christopherson wrote:
>> On Thu, Sep 08, 2022, Gerd Hoffmann wrote:

...

>> >  arch/x86/include/uapi/asm/kvm_para.h | 3 ++-
>> >  1 file changed, 2 insertions(+), 1 deletion(-)
>> > 
>> > diff --git a/arch/x86/include/uapi/asm/kvm_para.h b/arch/x86/include/uapi/asm/kvm_para.h
>> > index 6e64b27b2c1e..115bb34413cf 100644
>> > --- a/arch/x86/include/uapi/asm/kvm_para.h
>> > +++ b/arch/x86/include/uapi/asm/kvm_para.h
>> > @@ -37,7 +37,8 @@
>> >  #define KVM_FEATURE_HC_MAP_GPA_RANGE	16
>> >  #define KVM_FEATURE_MIGRATION_CONTROL	17
>> >  
>> > -#define KVM_HINTS_REALTIME      0
>> > +#define KVM_HINTS_REALTIME                      0
>> > +#define KVM_HINTS_PHYS_ADDRESS_SIZE_DATA_VALID  1
>> 
>> Why does KVM need to get involved?  This is purely a userspace problem.
>
> It doesn't.  I only need reserve a hints bit, and the canonical source
> for that happens to live in the kernel.  That's why this patch doesn't
> touch any actual code ;)
>
>> E.g. why not use QEMU's fw_cfg to communicate this information to the
>> guest?
>
> That is indeed the other obvious way to implement this.  Given this
> information will be needed in code paths which already do CPUID queries
> using CPUID to transport that information looked like the better option
> to me.

While this certainly looks like an overkill here, we could probably add
new, VMM-spefific CPUID leaves to KVM, e.g.

0x4000000A: VMM signature
0x4000000B: VMM features
0x4000000C: VMM quirks
...

this way VMMs (like QEMU) could identify themselves and suggest VMM
specific things to guests without KVM's involvement. Just if 'fw_cfg' is
not enough)

-- 
Vitaly