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
