Re: kexec, x86: Need a new e820 type support for kexec

[Toshi Kani <toshi.kani@hp.com>]

On Tue, 2015-08-18 at 16:34 +0800, Baoquan He wrote:
> Hi Toshi,
> 
> Sorry for replying late.
> 
> On 08/06/15 at 07:13pm, Toshi Kani wrote:
> > On Thu, 2015-08-06 at 16:12 +0800, Baoquan He wrote:
> > > Hi Toshi,
> > > 
> > > Does this patch work for you?
> > 
> > Hi Baoquan,
> > 
> > I have tested the patch with both E820_PMEM and E820_PRAM setups, and
> > confirmed it works fine for both cases. :-)  I did multiple kexec 
> > reboots
> > followed by a kdump in my testing.  So, please feel free to add:
> >  
> > Tested-by: Toshi Kani <toshi.kani@hp.com>
> 
> Thanks for testing, I will repost with Tested-by info.
> 
> > 
> > > There are things I am not sure. When jump to kexec/kdump kernel is 
> > > this
> > > PMEM still needed by system? 
> > 
> > Yes, after a kexec reboot, the kernel needs to be able to use NVDIMM as
> > before.  While the kernel actually uses NFIT table, not e820, the range
> > should be marked as PMEM for consistency.  The same goes to kdump kernel
> > since NVDIMM may be used as a dump device in future.
> > 
> > > And what's the difference between PRAM and
> > > PMEM? I saw in kernel commit ec776ef6 it introduced E820_PRAM for the
> > > non-standard protected e820 type, then in kernel commit ad5fb870 it
> > > introduced E820_PMEM for ACPI 6.0 persistent memory types. While it
> > > doesn't add complete support for E820_PMEM like E820_PRAM if I
> > > understand it correctly.
> > 
> > ACPI 6.0 spec defines E820_PMEM, which is used for NVDIMM devices from 
> > now
> > on.  ACPI 6.0 also defines NFIT table for NVDIMM along with this type.
> > 
> > Before these are defined in ACPI, E820_PRAM type was "unofficially" used 
> > by
> > some NVDIMM devices.  So, E820_PRAM was added for such legacy NVDIMMs. 
> >  Since the E820_PRAM case is very simple (it does not have any other FW
> > tables), it can be easily emulated with the "memmap=nn!ss" option.  So,
> > people may use the memmap option to emulate this legacy NVDIMM.    
> 
> I was wrong. In fact in kexec-tools memory info can be passed to kdump
> kernel by 2 ways. One is using memmap by specifying
> --pass-memmap-cmdline. The other one is storing memory regions in
> e820_map of real mode data structure by default. And the 1st way is
> rarely used. So no need to worry about the "memmap=nn!ss" option.
> 
> Since kernel parse_memmap_one doesn't support E820_PMEM well, I would
> like to ignore the PMEM adding in memmap way. So this patch is enough.

Yes, that is fine.

> > >  In this patch I simply pass E820_PMEM to kdump
> > > kernel as E820_PRAM when it emerges since kernel can parse E820_PRAM
> > > only in parse_memmap_one(), otherwise E820_PMEM has to be discarded or
> > > need be passed as E820_RESERVED. What do you think about this, need
> > > E820_PMEM be differentiated with E820_PRAM strictly? If yes, I think a
> > > kernel patch need be posted to fix this. If not, this patch is enough
> > > for supporting both of them in kexec.
> > 
> > E820_PMEM cannot be emulated by the "memmap=" option.  Do you have to 
> > use the "memmap=" options to pass the ranges for kdump kernel?  If so, 
> > I'd rather ignore E820_PMEM and let it be passed as E820_RESERVED.  The
> > kdump kernel can still obtain the info from NFIT if necessary.
> > 
> > As for the code change...
> > 
> > > @@ -640,6 +644,8 @@ static void cmdline_add_memmap_internal(char 
> > > *cmdline, 
> > > unsigned long startk,
> > >  		strcat (str_mmap, "K$");
> > >  	else if (type == RANGE_ACPI || type == RANGE_ACPI_NVS)
> > >  		strcat (str_mmap, "K#");
> > > +	else if (type == RANGE_PMEM || type == RANGE_PRAM)
> > > +		strcat (str_mmap, "K!");
> > 
> > It should only check with RANGE_PRAM, but I do not think this change 
> > matters much unless you also modify the caller cmdline_add_memmap(), 
> > which has the following check to skip other types.  I do not think we 
> > will use legacy NVDIMM device as a dump device, so you may ignore 
> > RANGE_PRAM and let it be passed as RESERVED as well (which is likely the > > case I tested with).
> > 
> >                 /* Only adding memory regions of RAM and ACPI */
> >                 if (type != RANGE_RAM &&
> >                     type != RANGE_ACPI &&
> >                     type != RANGE_ACPI_NVS)
> >                         continue;
> 
> Then if ignore PMEM adding into memmap, cmdline_add_memmap need not be
> cared any more.

Sounds good.

Thanks,
-Toshi