| Message ID | 20231211150520.299768-1-stewart.hildebrand@amd.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | xen/pci: fix phantom error path in assign_device() | expand |
On 11.12.2023 16:05, Stewart Hildebrand wrote: > Currently if an iommu_call() for a phantom function fails, there is no > indication of the failure. Propagate (but don't return) the error code > from the most recently failed iommu_call() and emit a warning. While > here, add a comment to clarify that the loop keeps iterating even when > failure is encountered. > > Fixes: cd7dedad8209 ("passthrough: simplify locking and logging") > Signed-off-by: Stewart Hildebrand <stewart.hildebrand@amd.com> > --- > Unlike assign_device(), deassign_device() breaks out of the phantom loop > when a failure is encountered and returns the error code. I'm curious > why assign_device() and deassign_device() behave differently? It looks > like it's been that way since > 4e9950dc1bd2 ("IOMMU: add phantom function support"). I was initially > inclined to break out of the loop and return the error code in > assign_device(), but adhering to the principle of Chesterton's fence, > I'd first like to understand why this is not currently being done. It's been a long time, but seeing the same pattern for add/remove I think the idea was that a device may still work with its phantom functions not properly mapped in the IOMMU, whereas failure to unmap means a domain may retain (partial) access to a device. > I'm aware that I could have avoided introducing a tmp local variable by > using the conditional operator with omitted middle operand: > > rc_nonfatal = iommu_call(...) ?: rc_nonfatal; > > However, I explicitly chose not to do this to avoid relying on a GNU > extension in yet another place. Introducing a helper variable is certainly okay, if you think that's better. However, in cases like ... > --- a/xen/drivers/passthrough/pci.c > +++ b/xen/drivers/passthrough/pci.c > @@ -1407,7 +1407,7 @@ static int assign_device(struct domain *d, u16 seg, u8 bus, u8 devfn, u32 flag) > { > const struct domain_iommu *hd = dom_iommu(d); > struct pci_dev *pdev; > - int rc = 0; > + int rc = 0, rc_nonfatal = 0; > > if ( !is_iommu_enabled(d) ) > return 0; > @@ -1443,21 +1443,28 @@ static int assign_device(struct domain *d, u16 seg, u8 bus, u8 devfn, u32 flag) > pci_to_dev(pdev), flag)) ) > goto done; > > - for ( ; pdev->phantom_stride; rc = 0 ) > + while ( pdev->phantom_stride ) > { > + int tmp; > + > devfn += pdev->phantom_stride; > if ( PCI_SLOT(devfn) != PCI_SLOT(pdev->devfn) ) > + { > + devfn -= pdev->phantom_stride; /* Adjust for printing */ > break; > + } > - rc = iommu_call(hd->platform_ops, assign_device, d, devfn, > - pci_to_dev(pdev), flag); > + tmp = iommu_call(hd->platform_ops, assign_device, d, devfn, > + pci_to_dev(pdev), flag); > + rc_nonfatal = tmp ? tmp : rc_nonfatal; ... this, I'm relatively certain most maintainers would agree that using the extension will yield easier to read code. Plus there's no reason to avoid extensions we use widely anyway, as long as there's no (reasonably neat) way to express the same without using extensions. Jan > + /* Keep iterating even if the iommu call failed. */ > } > > done: > - if ( rc ) > + if ( rc || rc_nonfatal ) > printk(XENLOG_G_WARNING "%pd: assign (%pp) failed (%d)\n", > - d, &PCI_SBDF(seg, bus, devfn), rc); > + d, &PCI_SBDF(seg, bus, devfn), rc ? rc : rc_nonfatal); > /* The device is assigned to dom_io so mark it as quarantined */ > - else if ( d == dom_io ) > + if ( !rc && d == dom_io ) > pdev->quarantine = true; > > return rc; > > base-commit: 1403131596fa77663708f6baa0fee8bf7b95eb5a
On 12/11/23 11:21, Jan Beulich wrote: > On 11.12.2023 16:05, Stewart Hildebrand wrote: >> Currently if an iommu_call() for a phantom function fails, there is no >> indication of the failure. Propagate (but don't return) the error code >> from the most recently failed iommu_call() and emit a warning. While >> here, add a comment to clarify that the loop keeps iterating even when >> failure is encountered. >> >> Fixes: cd7dedad8209 ("passthrough: simplify locking and logging") >> Signed-off-by: Stewart Hildebrand <stewart.hildebrand@amd.com> >> --- >> Unlike assign_device(), deassign_device() breaks out of the phantom loop >> when a failure is encountered and returns the error code. I'm curious >> why assign_device() and deassign_device() behave differently? It looks >> like it's been that way since >> 4e9950dc1bd2 ("IOMMU: add phantom function support"). I was initially >> inclined to break out of the loop and return the error code in >> assign_device(), but adhering to the principle of Chesterton's fence, >> I'd first like to understand why this is not currently being done. > > It's been a long time, but seeing the same pattern for add/remove I think > the idea was that a device may still work with its phantom functions not > properly mapped in the IOMMU, whereas failure to unmap means a domain may > retain (partial) access to a device. Thanks for the info. I'll add something about this to the in-code comment (see below). > >> I'm aware that I could have avoided introducing a tmp local variable by >> using the conditional operator with omitted middle operand: >> >> rc_nonfatal = iommu_call(...) ?: rc_nonfatal; >> >> However, I explicitly chose not to do this to avoid relying on a GNU >> extension in yet another place. > > Introducing a helper variable is certainly okay, if you think that's > better. However, in cases like ... > >> --- a/xen/drivers/passthrough/pci.c >> +++ b/xen/drivers/passthrough/pci.c >> @@ -1407,7 +1407,7 @@ static int assign_device(struct domain *d, u16 seg, u8 bus, u8 devfn, u32 flag) >> { >> const struct domain_iommu *hd = dom_iommu(d); >> struct pci_dev *pdev; >> - int rc = 0; >> + int rc = 0, rc_nonfatal = 0; >> >> if ( !is_iommu_enabled(d) ) >> return 0; >> @@ -1443,21 +1443,28 @@ static int assign_device(struct domain *d, u16 seg, u8 bus, u8 devfn, u32 flag) >> pci_to_dev(pdev), flag)) ) >> goto done; >> >> - for ( ; pdev->phantom_stride; rc = 0 ) >> + while ( pdev->phantom_stride ) >> { >> + int tmp; >> + >> devfn += pdev->phantom_stride; >> if ( PCI_SLOT(devfn) != PCI_SLOT(pdev->devfn) ) >> + { >> + devfn -= pdev->phantom_stride; /* Adjust for printing */ >> break; >> + } >> - rc = iommu_call(hd->platform_ops, assign_device, d, devfn, >> - pci_to_dev(pdev), flag); >> + tmp = iommu_call(hd->platform_ops, assign_device, d, devfn, >> + pci_to_dev(pdev), flag); >> + rc_nonfatal = tmp ? tmp : rc_nonfatal; > > ... this, I'm relatively certain most maintainers would agree that using > the extension will yield easier to read code. Plus there's no reason to > avoid extensions we use widely anyway, as long as there's no (reasonably > neat) way to express the same without using extensions. OK, I'll use the extension. > > Jan > >> + /* Keep iterating even if the iommu call failed. */ I'll change this in-code comment to: /* * Keep going in case of iommu_call failure for phantom functions. The * device may still be usable without phantom functions mapped in the * IOMMU. */ >> } >> >> done: >> - if ( rc ) >> + if ( rc || rc_nonfatal ) >> printk(XENLOG_G_WARNING "%pd: assign (%pp) failed (%d)\n", >> - d, &PCI_SBDF(seg, bus, devfn), rc); >> + d, &PCI_SBDF(seg, bus, devfn), rc ? rc : rc_nonfatal); >> /* The device is assigned to dom_io so mark it as quarantined */ >> - else if ( d == dom_io ) >> + if ( !rc && d == dom_io ) >> pdev->quarantine = true; >> >> return rc; >> >> base-commit: 1403131596fa77663708f6baa0fee8bf7b95eb5a >
diff --git a/xen/drivers/passthrough/pci.c b/xen/drivers/passthrough/pci.c index 28ed8ea8172a..71072efceb7a 100644 --- a/xen/drivers/passthrough/pci.c +++ b/xen/drivers/passthrough/pci.c @@ -1407,7 +1407,7 @@ static int assign_device(struct domain *d, u16 seg, u8 bus, u8 devfn, u32 flag) { const struct domain_iommu *hd = dom_iommu(d); struct pci_dev *pdev; - int rc = 0; + int rc = 0, rc_nonfatal = 0; if ( !is_iommu_enabled(d) ) return 0; @@ -1443,21 +1443,28 @@ static int assign_device(struct domain *d, u16 seg, u8 bus, u8 devfn, u32 flag) pci_to_dev(pdev), flag)) ) goto done; - for ( ; pdev->phantom_stride; rc = 0 ) + while ( pdev->phantom_stride ) { + int tmp; + devfn += pdev->phantom_stride; if ( PCI_SLOT(devfn) != PCI_SLOT(pdev->devfn) ) + { + devfn -= pdev->phantom_stride; /* Adjust for printing */ break; + } - rc = iommu_call(hd->platform_ops, assign_device, d, devfn, - pci_to_dev(pdev), flag); + tmp = iommu_call(hd->platform_ops, assign_device, d, devfn, + pci_to_dev(pdev), flag); + rc_nonfatal = tmp ? tmp : rc_nonfatal; + /* Keep iterating even if the iommu call failed. */ } done: - if ( rc ) + if ( rc || rc_nonfatal ) printk(XENLOG_G_WARNING "%pd: assign (%pp) failed (%d)\n", - d, &PCI_SBDF(seg, bus, devfn), rc); + d, &PCI_SBDF(seg, bus, devfn), rc ? rc : rc_nonfatal); /* The device is assigned to dom_io so mark it as quarantined */ - else if ( d == dom_io ) + if ( !rc && d == dom_io ) pdev->quarantine = true; return rc;
Currently if an iommu_call() for a phantom function fails, there is no indication of the failure. Propagate (but don't return) the error code from the most recently failed iommu_call() and emit a warning. While here, add a comment to clarify that the loop keeps iterating even when failure is encountered. Fixes: cd7dedad8209 ("passthrough: simplify locking and logging") Signed-off-by: Stewart Hildebrand <stewart.hildebrand@amd.com> --- Unlike assign_device(), deassign_device() breaks out of the phantom loop when a failure is encountered and returns the error code. I'm curious why assign_device() and deassign_device() behave differently? It looks like it's been that way since 4e9950dc1bd2 ("IOMMU: add phantom function support"). I was initially inclined to break out of the loop and return the error code in assign_device(), but adhering to the principle of Chesterton's fence, I'd first like to understand why this is not currently being done. I'm aware that I could have avoided introducing a tmp local variable by using the conditional operator with omitted middle operand: rc_nonfatal = iommu_call(...) ?: rc_nonfatal; However, I explicitly chose not to do this to avoid relying on a GNU extension in yet another place. --- xen/drivers/passthrough/pci.c | 21 ++++++++++++++------- 1 file changed, 14 insertions(+), 7 deletions(-) base-commit: 1403131596fa77663708f6baa0fee8bf7b95eb5a