Re: [PATCH 3/3] x86/pci: Fix racy accesses to MSI-X Control register

[David Vrabel <dvrabel@xxxxxxxxxx>]

On 12/12/2022 17:04, Jan Beulich wrote:
> On 10.11.2022 17:59, David Vrabel wrote:
> > --- a/xen/arch/x86/include/asm/msi.h
> > +++ b/xen/arch/x86/include/asm/msi.h
> > @@ -237,7 +237,10 @@ struct arch_msix {
> >       int table_refcnt[MAX_MSIX_TABLE_PAGES];
> >       int table_idx[MAX_MSIX_TABLE_PAGES];
> >       spinlock_t table_lock;
> > +    spinlock_t control_lock;
> >       bool host_maskall, guest_maskall;
> > +    uint16_t host_enable;
> If you want to keep this more narrow than "unsigned int", then please
> add a BUILD_BUG_ON() against NR_CPUS, so the need to update the field
> can be easily noticed (in some perhaps distant future).
This is only incremented:

- while holding the pci_devs lock, or
- while holding a lock for one of the associated IRQs.

Since there are at most 4096 MSI-X vectors (and thus at most 4096 IRQs), 
the highest value this can be (even with >> 4096 PCPUs) is 4097, thus a 
uint16_t is fine.
> > +static void msix_update_unlock(struct pci_dev *dev, unsigned int pos, uint16_t 
> > control)
> > +{
> > +    uint16_t new_control;
> > +    unsigned long flags;
> > +
> > +    spin_lock_irqsave(&dev->msix->control_lock, flags);
> > +
> > +    dev->msix->host_enable--;
> > +
> > +    new_control = control & ~(PCI_MSIX_FLAGS_ENABLE | PCI_MSIX_FLAGS_MASKALL);
> > +
> > +    if ( dev->msix->host_enable || dev->msix->guest_enable )
> > +        new_control |= PCI_MSIX_FLAGS_ENABLE;
> > +    if ( dev->msix->host_maskall || dev->msix->guest_maskall || 
> > dev->msix->host_enable )
> > +        new_control |= PCI_MSIX_FLAGS_MASKALL;
> In particular this use of "host_enable" suggests the field wants to be
> named differently: It makes no sense to derive MASKALL from ENABLE
> without it being clear (from the name) that the field is an init-time
> override only.
I think the name as-is makes sense. It is analogous to the host_maskall 
and complements guest_enable. I can't think of a better name, so what 
name do you suggest.
David