Re: [PATCH v1 00/11] fs/proc/vmcore: kdump support for virtio-mem on s390

[Baoquan He <bhe@redhat.com>]

On 11/15/24 at 09:55am, David Hildenbrand wrote:
> On 15.11.24 09:46, Baoquan He wrote:
> > On 10/25/24 at 05:11pm, David Hildenbrand wrote:
> > > This is based on "[PATCH v3 0/7] virtio-mem: s390 support" [1], which adds
> > > virtio-mem support on s390.
> > > 
> > > The only "different than everything else" thing about virtio-mem on s390
> > > is kdump: The crash (2nd) kernel allocates+prepares the elfcore hdr
> > > during fs_init()->vmcore_init()->elfcorehdr_alloc(). Consequently, the
> > > crash kernel must detect memory ranges of the crashed/panicked kernel to
> > > include via PT_LOAD in the vmcore.
> > > 
> > > On other architectures, all RAM regions (boot + hotplugged) can easily be
> > > observed on the old (to crash) kernel (e.g., using /proc/iomem) to create
> > > the elfcore hdr.
> > > 
> > > On s390, information about "ordinary" memory (heh, "storage") can be
> > > obtained by querying the hypervisor/ultravisor via SCLP/diag260, and
> > > that information is stored early during boot in the "physmem" memblock
> > > data structure.
> > > 
> > > But virtio-mem memory is always detected by as device driver, which is
> > > usually build as a module. So in the crash kernel, this memory can only be
> >                                         ~~~~~~~~~~~
> >                                         Is it 1st kernel or 2nd kernel?
> > Usually we call the 1st kernel as panicked kernel, crashed kernel, the
> > 2nd kernel as kdump kernel.
> 
> It should have been called "kdump (2nd) kernel" here indeed.
> 
> > > properly detected once the virtio-mem driver started up.
> > > 
> > > The virtio-mem driver already supports the "kdump mode", where it won't
> > > hotplug any memory but instead queries the device to implement the
> > > pfn_is_ram() callback, to avoid reading unplugged memory holes when reading
> > > the vmcore.
> > > 
> > > With this series, if the virtio-mem driver is included in the kdump
> > > initrd -- which dracut already takes care of under Fedora/RHEL -- it will
> > > now detect the device RAM ranges on s390 once it probes the devices, to add
> > > them to the vmcore using the same callback mechanism we already have for
> > > pfn_is_ram().
> > 
> > Do you mean on s390 virtio-mem memory region will be detected and added
> > to vmcore in kdump kernel when virtio-mem driver is initialized? Not
> > sure if I understand it correctly.
> 
> Yes exactly. In the kdump kernel, the driver gets probed and registers the
> vmcore callbacks. From there, we detect and add the device regions.

I see now, thanks for your confirmation.