Re: [Qemu-devel] [PATCH] pci/pcie: don't assume cap id 0 is reserved

[Alex Williamson <alex.williamson@redhat.com>]

On Thu, 16 Feb 2017 10:35:28 +0800
Peter Xu <peterx@redhat.com> wrote:

> On Wed, Feb 15, 2017 at 10:49:47PM +0200, Michael S. Tsirkin wrote:
> > VFIO actually wants to create a capability with ID == 0.
> > This is done to make guest drivers skip the given capability.
> > pcie_add_capability then trips up on this capability
> > when looking for end of capability list.
> > 
> > To support this use-case, it's easy enough to switch to
> > e.g. 0xffffffff for these comparisons - we can be sure
> > it will never match a 16-bit capability ID.
> > 
> > Signed-off-by: Michael S. Tsirkin <mst@redhat.com>  
> 
> Reviewed-by: Peter Xu <peterx@redhat.com>
> 
> Two nits:
> 
> (1) maybe we can s/0xffffffff/0xffff/ in the whole patch since ecap_id
>     is 16 bits

The former is used because it's beyond the address space of a valid
capability.  Using 0xffff just makes the situation different, not
better.

> 
> (2) maybe we can add one more sentence in the comment below showing
>     where the 0xffff thing comes from (it comes from PCIe spec 7.9.2)

The capability in hardware is 16bits, thus a value that exceeds 16 bits
can never match a valid ID.  It has nothing to do with 7.9.2.  Thanks,

Alex

> > ---
> >  hw/pci/pcie.c | 11 +++++++----
> >  1 file changed, 7 insertions(+), 4 deletions(-)
> > 
> > diff --git a/hw/pci/pcie.c b/hw/pci/pcie.c
> > index cbd4bb4..f4dd177 100644
> > --- a/hw/pci/pcie.c
> > +++ b/hw/pci/pcie.c
> > @@ -610,7 +610,8 @@ bool pcie_cap_is_arifwd_enabled(const PCIDevice *dev)
> >   * uint16_t ext_cap_size
> >   */
> >  
> > -static uint16_t pcie_find_capability_list(PCIDevice *dev, uint16_t cap_id,
> > +/* Passing a cap_id value > 0xffff will return 0 and put end of list in prev */
> > +static uint16_t pcie_find_capability_list(PCIDevice *dev, uint32_t cap_id,
> >                                            uint16_t *prev_p)
> >  {
> >      uint16_t prev = 0;
> > @@ -679,9 +680,11 @@ void pcie_add_capability(PCIDevice *dev,
> >      } else {
> >          uint16_t prev;
> >  
> > -        /* 0 is reserved cap id. use internally to find the last capability
> > -           in the linked list */
> > -        next = pcie_find_capability_list(dev, 0, &prev);
> > +        /*
> > +         * 0xffffffff is not a valid cap id (it's a 16 bit field). use
> > +         * internally to find the last capability in the linked list.
> > +         */
> > +        next = pcie_find_capability_list(dev, 0xffffffff, &prev);
> >  
> >          assert(prev >= PCI_CONFIG_SPACE_SIZE);
> >          assert(next == 0);
> > -- 
> > MST  
> 
> -- peterx