linux-kernel - Re: [PATCH] kvm/x86: reserve bit KVM_HINTS_PHYS_ADDRESS_SIZE_DATA

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Date:   Tue, 13 Sep 2022 11:40:30 +0200
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     Gerd Hoffmann <kraxel@...hat.com>, kvm@...r.kernel.org,
        Paolo Bonzini <pbonzini@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        "open list:X86 ARCHITECTURE (32-BIT AND 64-BIT)" 
        <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] kvm/x86: reserve bit
 KVM_HINTS_PHYS_ADDRESS_SIZE_DATA_VALID

Sean Christopherson <seanjc@...gle.com> writes:

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

...

>> 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).

True, VMM can just use another hypervisor space (+0x100) but we can view
it from a slightly different angle: KVM itself is insufficient to run
VMs, there's always a VMM in the background, we may want to provide a
"standard" for its information meaning guests won't need to search for
VMMs signature[s] but can directly refer to "standardized" leaves. 

(All this is a purely theoretical discussion at this point, we need a
good reason to introduce this first).

-- 
Vitaly