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    SubjectRe: [PATCH] kvm/x86: reserve bit KVM_HINTS_PHYS_ADDRESS_SIZE_DATA_VALID
    On Fri, Sep 09, 2022, Vitaly Kuznetsov wrote:
    > Gerd Hoffmann <kraxel@redhat.com> 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).

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