On Mon, 25 Jul 2016, Julien Grall wrote: > On 25/07/16 14:39, Vitaly Kuznetsov wrote: > > Julien Grall <julien.gr...@arm.com> writes: > > > > > Hi David, > > > > > > On 25/07/16 13:38, David Vrabel wrote: > > > > On 30/06/16 16:56, Vitaly Kuznetsov wrote: > > > > > It may happen that Xen's and Linux's ideas of vCPU id diverge. In > > > > > particular, when we crash on a secondary vCPU we may want to do kdump > > > > > and unlike plain kexec where we do migrate_to_reboot_cpu() we try > > > > > booting > > > > > on the vCPU which crashed. This doesn't work very well for PVHVM > > > > > guests as > > > > > we have a number of hypercalls where we pass vCPU id as a parameter. > > > > > These > > > > > hypercalls either fail or do something unexpected. To solve the issue > > > > > we > > > > > need to have a mapping between Linux's and Xen's vCPU ids. > > > > > > > > > > This series solves the issue for x86 PVHVM guests. PV guests don't > > > > > (and > > > > > probably won't) support kdump so I always assume Xen's vCPU id == > > > > > Linux's > > > > > vCPU id. ARM guests will probably need to get proper mapping once we > > > > > start > > > > > supporting kexec/kdump there. > > > > > > > > Applied to for-linus-4.8, thanks. > > > > > > It would have been nice to send a ping before applying. This patch > > > series is containing Xen ARM code which has not been acked by Stefano, > > > nor had feedback from ARM side. > > > > > > For instance given that all the hypercalls are representing a "vcpu > > > id" using "uint32_t" it is a bit weird to use "int" to define > > > xen_vcpu_id (see patch #3). > > > > CPU id is usually 'int' in linux and now we pass it to all > > hypercalls as it is. > > Well, we need to differentiate between the internal representation of the CPU > which is based on the boot order and the logical CPU ID. For instance on ARM, > the logical CPU ID may not be contiguous nor 0 for the first CPU. > > From my understanding, the macros in patch #3 will be used at the last minute > when prepare the hypercall data. IHMO this is very similar to a logical ID and > defined as uint32_t by the hypercall ABI. > > Although, I agree that currently we use the internal CPU id on ARM which is > very unfortunate because this value is based on the order of the nodes in the > device tree. > > One way to abolish it on ARM would be to use the MPIDR (or at least a part) > for the VCPU ID.
That's a good idea -- much more reliable than the device tree cpu order.