Re: [Xen-devel] [PATCH v6 02/11] vpci: introduce basic handlers to trap accesses to the PCI config space

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 04.10.17 at 12:32, <roger.pau@xxxxxxxxxx> wrote:
> On Wed, Oct 04, 2017 at 09:54:18AM +0000, Jan Beulich wrote:
>> >>> On 04.10.17 at 11:24, <roger.pau@xxxxxxxxxx> wrote:
>> > On Wed, Oct 04, 2017 at 08:30:38AM +0000, Jan Beulich wrote:
>> >> >>> On 19.09.17 at 17:29, <roger.pau@xxxxxxxxxx> wrote:
>> >> > +static int vpci_portio_read(const struct hvm_io_handler *handler,
>> >> > +                            uint64_t addr, uint32_t size, uint64_t 
>> >> > *data)
>> >> > +{
>> >> > +    struct domain *d = current->domain;
>> >> > +    unsigned int reg;
>> >> > +    pci_sbdf_t sbdf;
>> >> > +    uint32_t cf8;
>> >> > +
>> >> > +    *data = ~(uint64_t)0;
>> >> > +
>> >> > +    if ( addr == 0xcf8 )
>> >> > +    {
>> >> > +        ASSERT(size == 4);
>> >> > +        *data = d->arch.hvm_domain.pci_cf8;
>> >> > +        return X86EMUL_OKAY;
>> >> > +    }
>> >> > +
>> >> > +    cf8 = ACCESS_ONCE(d->arch.hvm_domain.pci_cf8);
>> >> > +    if ( !CF8_ENABLED(cf8) )
>> >> > +        return X86EMUL_OKAY;
>> >> 
>> >> Why is this OKAY instead of UNHANDLEABLE? The access is supposed to be
>> >> forwarded to qemu if it's not a config space one. Same in the write path
>> >> then.
>> > 
>> > No, I don't think this should be forwarded to QEMU. It is a config
>> > space access (because vpci_portio_accept returned true). But the value
>> > in CF8 doesn't have the enabled bit set, hence the access is
>> > discarded.
>> 
>> With the enable bit clear it is my understanding that this is then
>> _not_ a config space access. vpci_portio_accept() simply doesn't
>> have enough information to tell.
> 
> OK, it was my understanding that accesses to cf8/cfc where only used
> by the PCI config space.

Just like with the overlaid byte accesses to port cf9, other such
overlays could exist too; iirc back when PCI was introduced the
enable bit was used to make sure other uses of this port range
remained reasonably unaffected.

Jan


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