On Wed, Jul 09, 2025 at 05:34:28AM +0000, Chen, Jiqian wrote: > On 2025/7/9 13:32, Jan Beulich wrote: > > On 09.07.2025 07:29, Chen, Jiqian wrote: > >> On 2025/7/8 22:10, Jan Beulich wrote: > >>> On 04.07.2025 09:07, Jiqian Chen wrote: > >>>> --- a/xen/drivers/vpci/header.c > >>>> +++ b/xen/drivers/vpci/header.c > >>>> @@ -836,6 +836,39 @@ static int vpci_init_capability_list(struct pci_dev > >>>> *pdev) > >>>> PCI_STATUS_RSVDZ_MASK); > >>>> } > >>>> > >>>> +static int vpci_init_ext_capability_list(const struct pci_dev *pdev) > >>>> +{ > >>>> + unsigned int pos = PCI_CFG_SPACE_SIZE; > >>>> + > >>>> + if ( !is_hardware_domain(pdev->domain) ) > >>>> + /* Extended capabilities read as zero, write ignore for DomU */ > >>>> + return vpci_add_register(pdev->vpci, vpci_read_val, NULL, > >>>> + pos, 4, (void *)0); > >>>> + > >>>> + do > >>>> + { > >>>> + uint32_t header = pci_conf_read32(pdev->sbdf, pos); > >>>> + int rc; > >>>> + > >>>> + rc = vpci_add_register(pdev->vpci, vpci_read_val, > >>>> vpci_hw_write32, > >>>> + pos, 4, (void *)(uintptr_t)header); > >>> > >>> If it wasn't for this use of vpci_hw_write32(), I'd be happy to provide my > >>> R-b. But this continues to look bogus to me: What use is it to allow > >>> writes > >>> when Dom0 then can't read back any possible effect of such a write (in the > >>> unexpected event that some of the bits were indeed writable)? > >> Oh, I got your concern. > >> What do you think about updating the header value after writing to > >> hardware in write function? > > > That would imo be a layering violation. Once again that's something that you > > primarily would need Roger's input on. > OK, wait for Roger's input.
Hm, I see the asymmetry of allowing writes but not direct writes, my thought was to give the hw domain as less interference as possibly, hence my recommendation to use vpci_hw_write32(). I practice I think it's very unlikely that devices re-use reserved bits in the capability register, so I'm fine with using NULL (thus discarding the write). We can always add more complex handling here if we ever came across a device that requires it. Thanks, Roger.