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


From: Peter Xu
Subject: Re: [Qemu-devel] [PATCH] pci/pcie: don't assume cap id 0 is reserved
Date: Thu, 16 Feb 2017 11:04:46 +0800
User-agent: Mutt/1.5.24 (2015-08-30)

On Wed, Feb 15, 2017 at 07:52:35PM -0700, Alex Williamson wrote:
> On Thu, 16 Feb 2017 10:35:28 +0800
> Peter Xu <address@hidden> 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 <address@hidden>  
> > 
> > Reviewed-by: Peter Xu <address@hidden>
> > 
> > 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.

But isn't pcie_find_capability_list() defining cap_id parameter as
uint16_t? In that case, 0xffffffff will be the same as 0xffff since
we'll just take the lower 16 bits?

> 
> > 
> > (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
> 

-- peterx



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