Re: [Xen-devel] [PATCH-for-4.13] libxl: choose an appropriate default for passthrough...

[Ian Jackson <ian.jackson@xxxxxxxxxx>]

Paul Durrant writes ("[PATCH-for-4.13] libxl: choose an appropriate default for 
passthrough..."):
> ...if there is no IOMMU or it is globally disabled.
> 
> Without this patch, the following assertion may be hit:
> 
> xl: libxl_create.c:589: libxl__domain_make: Assertion `info->passthrough != 
> LIBXL_PASSTHROUGH_ENABLED' failed.
> 
> This is because libxl__domain_create_info_setdefault() currently only sets
> an appropriate value for 'passthrough' in the case that 'cap_hvm_directio'
> is true, which is not the case unless an IOMMU is present and enabled in
> the system. This is normally masked by xl choosing a default value, but
> that will not happen if xl is not used (e.g. when using libvirt) or when
> a stub domain is being created.

It's weird that after this patch "enabled" can mean DISABLED.  Surely
if you say `passthrough="enabled"' and the host has no PT support (eg
it's disabled in the bios) it should fail ?

Normally libxl config options have an "unknown" or "default" option.

Also it is anomalous that xl is doing the complex work of choosing a
default.  I think all the complex code

+    switch (c_info->passthrough) {
+    case LIBXL_PASSTHROUGH_ENABLED:

in xl_parse.c should be in libxl.  (Some of it is there already.)

I'm sorry that I wasn't didn't review babde47a3fed...

Ian.

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