linux-kernel - Re: [PATCH 0/3] KVM: x86: KVM_MEM_PCI

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Message-ID: <20200806081800-mutt-send-email-mst@kernel.org>
Date:   Thu, 6 Aug 2020 08:19:30 -0400
From:   "Michael S. Tsirkin" <mst@...hat.com>
To:     Vitaly Kuznetsov <vkuznets@...hat.com>
Cc:     kvm@...r.kernel.org, Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Peter Xu <peterx@...hat.com>,
        Julia Suvorova <jsuvorov@...hat.com>,
        Andy Lutomirski <luto@...nel.org>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 0/3] KVM: x86: KVM_MEM_PCI_HOLE memory

On Thu, Aug 06, 2020 at 01:39:09PM +0200, Vitaly Kuznetsov wrote:
> "Michael S. Tsirkin" <mst@...hat.com> writes:
> 
> > About the feature bit, I am not sure why it's really needed. A single
> > mmio access is cheaper than two io accesses anyway, right? So it makes
> > sense for a kvm guest whether host has this feature or not.
> > We need to be careful and limit to a specific QEMU implementation
> > to avoid tripping up bugs, but it seems more appropriate to
> > check it using pci host IDs.
> 
> Right, it's just that "running on KVM" is too coarse grained, we just
> need a way to somehow distinguish between "known/good" and
> "unknown/buggy" configurations.

Basically it's not KVM, it's QEMU that is known good.  QEMU vendor id in
the pci host seems like a reasonable way to detect that. If someone
reuses QEMU ID - I guess they better behave just like QEMU :)

I also proposed only limiting this to register 0 (device id),
will make it very unlikely this can break accidentally ...

> -- 
> Vitaly