Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
From:   Vitaly Kuznetsov <vkuznets@redhat.com>
To:     Gerd Hoffmann <kraxel@redhat.com>,
        Sean Christopherson <seanjc@google.com>
Cc:     kvm@vger.kernel.org, Paolo Bonzini <pbonzini@redhat.com>,
        Wanpeng Li <wanpengli@tencent.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@kernel.org>,
        "H. Peter Anvin" <hpa@zytor.com>,
        "open list:X86 ARCHITECTURE (32-BIT AND 64-BIT)" 
        <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH] kvm/x86: reserve bit
 KVM_HINTS_PHYS_ADDRESS_SIZE_DATA_VALID
In-Reply-To: <20220909050224.rzlt4x7tjrespw3k@sirius.home.kraxel.org>
References: <20220908114146.473630-1-kraxel@redhat.com>
 <YxoBtD+3sgEEiaFF@google.com>
 <20220909050224.rzlt4x7tjrespw3k@sirius.home.kraxel.org>
Date:   Fri, 09 Sep 2022 11:02:49 +0200
Message-ID: <87tu5grkcm.fsf@redhat.com>
MIME-Version: 1.0
Content-Type: text/plain
Precedence: bulk

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)

-- 
Vitaly