On Fri, Jan 05, 2024 at 10:06:13 -0500, Laine Stump wrote:
> On 1/5/24 8:46 AM, Peter Krempa wrote:
> > On Fri, Jan 05, 2024 at 03:20:16 -0500, Laine Stump wrote:
> > > Rather than always binding to the vfio-pci driver, use the new
> > > function virPCIDeviceFindBestVFIOVariant() to see if the running
> > > kernel has a VFIO variant driver available that is a better match for
> > > the device, and if one is found, use that instead.
> > >
> > > virPCIDeviceFindBestVFIOVariant() function reads the modalias file for
> > > the given device from sysfs, then looks through
> > > /lib/modules/${kernel_release}/modules.alias for the vfio_pci alias
> > > that matches with the least number of wildcard ('*') fields.
> > >
> > > The appropriate "VFIO variant" driver for a device will be the PCI
> > > driver implemented by the discovered module - these drivers are
> > > compatible with (and provide the entire API of) the standard vfio-pci
> > > driver, but have additional device-specific APIs that can be useful
> > > for, e.g., saving/restoring state for migration.
> > >
> > > If a specific driver is named (using <driver model='blah'/> in the
> > > device XML), that will still be used rather than searching
> > > modules.alias; this makes it possible to force binding of vfio-pci if
> > > there is an issue with the auto-selected variant driver.
> > >
> > > Signed-off-by: Laine Stump <[email protected]>
> > > ---
> > >
> > > Changes from V2:
> > >
> > > * fail if device modalias file isn't found.
> >
> > This [1] ...
> >
> > > * use unsigned int instead of int for wildcardCt
> > > * increase file memory buffer from 4MB to 8MB
> > > * other minor nits pointed out by Peter
> >
> > [...]
> >
> > > +/* virPCIDeviceFindBestVFIOVariant:
> > > + *
> > > + * Find the "best" match of all vfio_pci aliases for @dev in the host
> > > + * modules.alias file. This uses the algorithm of finding every
> > > + * modules.alias line that begins with "vfio_pci:", then picking the
> > > + * one that matches the device's own modalias value (from the file of
> > > + * that name in the device's sysfs directory) with the fewest
> > > + * "wildcards" (* character, meaning "match any value for this
> > > + * attribute").
> > > + */
> > > +int
> > > +virPCIDeviceFindBestVFIOVariant(virPCIDevice *dev,
> > > + char **moduleName)
> > > +{
> > > + g_autofree char *devModAliasPath = NULL;
> > > + g_autofree char *devModAliasContent = NULL;
> > > + const char *devModAlias;
> > > + g_autoptr(virPCIDeviceAliasInfo) devModAliasInfo = NULL;
> > > + struct utsname unameInfo;
> > > + g_autofree char *modulesAliasPath = NULL;
> > > + g_autofree char *modulesAliasContent = NULL;
> > > + const char *line;
> > > + unsigned int currentBestWildcardCt = INT_MAX;
> >
> > UINT_MAX
> >
> > > +
> > > + *moduleName = NULL;
> > > +
> > > + /* get the modalias values for the device from sysfs */
> > > + devModAliasPath = virPCIFile(dev->name, "modalias");
> > > + if (virFileReadAll(devModAliasPath, 100, &devModAliasContent) < 0)
> >
> > ... [1] isn't true.
>
> How so? If the file can't be opened, then virFileReadAll logs an error and
> returns -1, and changed changed this code from ignoring that to also
> returning -1, which will cause the caller to fail. What am I missing?
I think I misremembered. I thought that the idea was that this should
not fail if the file is missing.
> > > + return -1;
> > > +
> > > + VIR_DEBUG("modalias path: '%s' contents: '%s'",
> > > + devModAliasPath, devModAliasContent);
> > > +
> > > + /* "pci:vNNNNNNNNdNNNNNNNNsvNNNNNNNNsdNNNNNNNNbcNNscNNiNN\n" */
> > > + if ((devModAlias = STRSKIP(devModAliasContent, "pci:")) == NULL ||
> > > + !(devModAliasInfo = virPCIDeviceAliasInfoNew(devModAlias))) {
> > > + virReportError(VIR_ERR_INTERNAL_ERROR,
> > > + _("device modalias file %1$s content has improper
> > > format"),
> > > + devModAliasPath);
> > > + return -1;
> > > + }
> > > +
> > > + uname(&unameInfo);
> > > + modulesAliasPath = g_strdup_printf("/lib/modules/%s/modules.alias",
> > > unameInfo.release);
> > > + if (virFileReadAll(modulesAliasPath, 8 * 1024 * 1024,
> > > &modulesAliasContent) < 0)
> > > + return -1;
> >
> > IIUC this is picking a driver which is 'better' than the default
> > 'vfio_pci', but the device itself would still work if the default were
> > used, right?
>
> Correct.
>
> >
> > In such case do we really want to treat any of the errors above as
> > failures?
>
> I was on the fence about this, which was why earlier versions ignored the
> error. But your earlier suggestion to check for file existence before trying
> to read the file (thus eliminating any sort of message at all, which I think
> would be wrong) pushed me in the direction of logging the error and failing
> (since it would be *really* broken for the modalias file to be missing).
>
> I would also be fine with logging the error when virFileReadAll() fails (ie
> *not* checking if the file exists so we could silently avoid the error log),
> but then clearing the and returning success. Does that work for you?
Hmmm, yeah I understand why. Each option has benefits.
I was worried a bit about short-term occuring bugs, but long term it
makes sense to just report teh error if it doesnt' work ...
> > I presueme a workaround for if any of the above breaks is to
> > use an explicitly specified driver, right?
>
> Correct. If a driver is explicitly given, then we don't attempt to find a
> better match. So in the case that a device really had no modalias file but
> was otherwise working, the error could be worked around by adding <driver
> model='vfio-pci'/> to the XML (assuming that whatever is creating the XML is
> able to do that).
... and there is a workaround for if stuff breaks. Thus:
Reviewed-by: Peter Krempa <[email protected]>
(fix the UINT_MAX thing)
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