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

[Sean Christopherson]

On Fri, Sep 09, 2022, Vitaly Kuznetsov wrote:
> 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)

I don't think KVM needs to get involved in that either.  The de facto hypervisor
CPUID standard already allows for multiple hypervisors/VMMs to announce themselves
to the guest, e.g. QEMU could add itself as another VMM using 0x40000100 (shifted
as necessary to accomodate KVM+Hyper-V).