Re: [PATCH V7 10/11] xen/arm: translate virtual PCI bus topology for guests

[Oleksandr <olekstysh@xxxxxxxxx>]

On 29.07.22 09:06, Jan Beulich wrote:

Hello Jan

> On 28.07.2022 18:35, Oleksandr wrote:
> > On 28.07.22 10:15, Jan Beulich wrote:
> > > On 27.07.2022 21:39, Oleksandr wrote:
> > > > On 27.07.22 20:54, Oleksandr wrote:
> > > > > On 26.07.22 18:16, Jan Beulich wrote:
> > > > > > On 19.07.2022 19:42, Oleksandr Tyshchenko wrote:
> > > > > > > --- a/xen/arch/arm/vpci.c
> > > > > > > +++ b/xen/arch/arm/vpci.c
> > > > > > > @@ -41,6 +41,16 @@ static int vpci_mmio_read(struct vcpu *v,
> > > > > > > mmio_info_t *info,
> > > > > > >         /* data is needed to prevent a pointer cast on 32bit */
> > > > > > >         unsigned long data;
> > > > > > >     +    /*
> > > > > > > +     * For the passed through devices we need to map their virtual
> > > > > > > SBDF
> > > > > > > +     * to the physical PCI device being passed through.
> > > > > > > +     */
> > > > > > > +    if ( !bridge && !vpci_translate_virtual_device(v->domain, &sbdf) )
> > > > > > > +    {
> > > > > > > +        *r = ~0ul;
> > > > > > > +        return 1;
> > > > > > > +    }
> > > > > > I'm probably simply lacking specific Arm-side knowledge, but it strikes
> > > > > > me as odd that the need for translation would be dependent upon
> > > > > > "bridge".
> > > > > I am afraid I cannot answer immediately.
> > > > >
> > > > > I will analyze that question and provide an answer later on.
> > > > Well, most likely that "valid" bridge pointer here is just used as an
> > > > indicator of hwdom currently, so no need to perform virt->phys
> > > > translation for sbdf.
> > > >
> > > > You can see that domain_vpci_init() passes a valid value for hwdom and
> > > > NULL for other domains when setting up vpci_mmio* callbacks.
> > > Oh, I see.
> > >
> > > > Alternatively, I guess we could use "!is_hardware_domain(v->domain)"
> > > > instead of "!bridge" in the first part of that check. Shall I?
> > > Maybe simply add a comment? Surely checking "bridge" is cheaper than
> > > using is_hardware_domain(), so I can see the benefit. But the larger
> > > arm/vpci.c grows, the less obvious the connection will be without a
> > > comment.
> > Agree the connection is worth a comment ...
> >
> >
> >
> > >    (Instead of a comment, an alternative may be a suitable
> > > assertion, which then documents the connection at the same time, e.g.
> > > ASSERT(!bridge == !is_hardware_domain(v->domain)). But that won't be
> > > possible in e.g. vpci_sbdf_from_gpa(), where apparently a similar
> > > assumption is being made.)
> >      ... or indeed to put such ASSERT _before_ vpci_sbdf_from_gpa().
> >
> > This will cover assumption being made in both places.
> >
> >
> > diff --git a/xen/arch/arm/vpci.c b/xen/arch/arm/vpci.c
> > index a9fc5817f9..1d4b1ef39e 100644
> > --- a/xen/arch/arm/vpci.c
> > +++ b/xen/arch/arm/vpci.c
> > @@ -37,10 +37,24 @@ static int vpci_mmio_read(struct vcpu *v,
> > mmio_info_t *info,
> >                              register_t *r, void *p)
> >    {
> >        struct pci_host_bridge *bridge = p;
> > -    pci_sbdf_t sbdf = vpci_sbdf_from_gpa(bridge, info->gpa);
> > +    pci_sbdf_t sbdf;
> >        /* data is needed to prevent a pointer cast on 32bit */
> >        unsigned long data;
> >
> > +    ASSERT(!bridge == !is_hardware_domain(v->domain));
> > +
> > +    sbdf = vpci_sbdf_from_gpa(bridge, info->gpa);
> > +
> > +    /*
> > +     * For the passed through devices we need to map their virtual SBDF
> > +     * to the physical PCI device being passed through.
> > +     */
> > +    if ( !bridge && !vpci_translate_virtual_device(v->domain, &sbdf) )
> > +    {
> > +        *r = ~0ul;
> > +        return 1;
> > +    }
> > +
> >        if ( vpci_ecam_read(sbdf, ECAM_REG_OFFSET(info->gpa),
> >                            1U << info->dabt.size, &data) )
> >        {
> > @@ -57,7 +71,18 @@ static int vpci_mmio_write(struct vcpu *v,
> > mmio_info_t *info,
> >                               register_t r, void *p)
> >    {
> >        struct pci_host_bridge *bridge = p;
> > -    pci_sbdf_t sbdf = vpci_sbdf_from_gpa(bridge, info->gpa);
> > +    pci_sbdf_t sbdf;
> > +
> > +    ASSERT(!bridge == !is_hardware_domain(v->domain));
> > +
> > +    sbdf = vpci_sbdf_from_gpa(bridge, info->gpa);
> > +
> > +    /*
> > +     * For the passed through devices we need to map their virtual SBDF
> > +     * to the physical PCI device being passed through.
> > +     */
> > +    if ( !bridge && !vpci_translate_virtual_device(v->domain, &sbdf) )
> > +        return 1;
> >
> >        return vpci_ecam_write(sbdf, ECAM_REG_OFFSET(info->gpa),
> >                               1U << info->dabt.size, r);
> > diff --git a/xen/drivers/vpci/vpci.c b/xen/drivers/vpci/vpci.c
> > index d4601ecf9b..fc2c51dc3e 100644
> >
> >
> > Any preference here?
> >
> >
> > Personally, I think that such ASSERT will better explain the connection
> > than the comment will do.
> Indeed I'd also prefer ASSERT()s being put there.
good


>   But my opinion is
> secondary here, as I'm not a maintainer of this code.

sure, let's see what the Arm maintainers will say


> Jan
--
Regards,

Oleksandr Tyshchenko