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Date:   Wed, 21 Sep 2022 15:00:03 +0000
From:   Sean Christopherson <seanjc@google.com>
To:     Gerd Hoffmann <kraxel@redhat.com>
Cc:     kvm@vger.kernel.org, Paolo Bonzini <pbonzini@redhat.com>,
        Wanpeng Li <wanpengli@tencent.com>,
        Vitaly Kuznetsov <vkuznets@redhat.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
Message-ID: <Yysm8/Nkt6EUI5+k@google.com>
References: <20220908114146.473630-1-kraxel@redhat.com>
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 <20220909050224.rzlt4x7tjrespw3k@sirius.home.kraxel.org>
 <20220921134246.xibospqoktp4wjie@sirius.home.kraxel.org>
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On Wed, Sep 21, 2022, Gerd Hoffmann wrote:
> On Fri, Sep 09, 2022 at 07:02:24AM +0200, Gerd Hoffmann wrote:
> > On Thu, Sep 08, 2022 at 02:52:36PM +0000, Sean Christopherson wrote:
> > > On Thu, Sep 08, 2022, Gerd Hoffmann wrote:
> > > > -#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 ;)

The issue is that this "hint" effectively breaks other VMMs that already provide
an accurate guest.MAXPHYADDR.

> > > 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.
> 
> I'd like to move forward with this.
> 
> So, any comment further comments and opinions?
> Is it ok to grab a hints bit given the explanation above?
> Or should I go for the fw_cfg approach?

My strong preference is the fw_cfg approach, or if the guest side really wants to
use CPUID, have QEMU define it's own CPUID signature and provide QEMU-specific
hints/quirks that way.