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Re: [Qemu-devel] [PATCH] vfio/pci: Hide SR-IOV capability

From: Eric Blake
Subject: Re: [Qemu-devel] [PATCH] vfio/pci: Hide SR-IOV capability
Date: Mon, 20 Jun 2016 16:23:07 -0600
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Thunderbird/38.3.0

On 06/20/2016 04:04 PM, Alex Williamson wrote:
> The kernel currently exposes the SR-IOV capability as read-only
> through vfio-pci.  This is sufficient to protect the host kernel, but
> has the potential to confuse guests without further virtualization.
> In particular, OVMF tries to size the VF BARs and comes up with absurd
> results, ending with an assert.  There's not much point in adding
> virtualization to a read-only capability, so we simply hide it for
> now.  If the kernel ever enables SR-IOV virtualization, we should
> easily be able to test it through VF BAR sizing or explicit flags.
> Testing whether we should parse extended capabilities is also pulled
> into the function to keep these assumptions in one place.
> Signed-off-by: Alex Williamson <address@hidden>
> ---

> +     * Extended capabilities are chained with each pointing to the next, so 
> we
> +     * can drop anything other than the head of the chain simply by modifying
> +     * the previous next pointer.  For the head of the chain, we can modify 
> the
> +     * capability ID to something that cannot match a valid capability.  ID
> +     * 0 is reserved for this since absence of capabilities is indicated by
> +     * 0 for the ID, version, AND next pointer.  However, 
> pcie_add_capability()
> +     * uses ID 0 as reserved for list management and will incorrectly match 
> and
> +     * assert if we attempt to pre-load the head of the chain with with this
> +     * ID.  Use ID 0xFFFF temporarily since it is also seems to be reserved 
> in
> +     * part for identifying abscense of capabilities in a root complex 
> register


Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org

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