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Re: [PATCH RESEND v2] block/nvme: introduce PMR support from NVMe 1.4 sp

From: Stefan Hajnoczi
Subject: Re: [PATCH RESEND v2] block/nvme: introduce PMR support from NVMe 1.4 spec
Date: Tue, 17 Mar 2020 11:23:20 +0000

On Mon, Mar 16, 2020 at 5:10 PM Andrzej Jakowski
<address@hidden> wrote:
> On 3/16/20 4:32 AM, Stefan Hajnoczi wrote:
> > On Wed, Mar 11, 2020 at 11:08:27PM -0700, Klaus Birkelund Jensen wrote:
> >> On Mar 11 15:54, Andrzej Jakowski wrote:
> >>> On 3/11/20 2:20 AM, Stefan Hajnoczi wrote:
> >>>> Please try:
> >>>>
> >>>>   $ git grep pmem
> >>>>
> >>>> backends/hostmem-file.c is the backend that can be used and the
> >>>> pmem_persist() API can be used to flush writes.
> >>> I've reworked this patch into hostmem-file type of backend.
> >>> From simple tests in virtual machine: writing to PMR region
> >>> and then reading from it after VM power cycle I have observed that
> >>> there is no persistency.
> > Sounds like an integration bug.  QEMU's NVDIMM emulation uses
> > HostMemoryBackend and file contents survive guest reboot.
> >
> > If you would like help debugging this, please post a link to the code
> > and the command-line that you are using.
> >
> Code is posted here
> https://github.com/AndrzejJakowski/qemu/commit/3a7762a1d13ff1543d1da430748eb24e38faab6f
> QEMU command line:
> # below are just relevant pieces of configuration, other stuff omitted
> # tried different setting (e.g. pmem=on and pmem=off)
> ./x86_64-softmmu/qemu-system-x86_64 ... \
> -object 
> memory-backend-file,id=mem1,share=off,pmem=on,mem-path=../nvme_pmr.bin,size=$((1*1024*1024))
>  \

share=off is MAP_PRIVATE.  If persistence is desired then share=on
should be used.

However, this shouldn't affect "system_reset" behavior since the QEMU
process still has the same mapped file open.

> -drive file=../nvme.bin,format=raw,if=none,id=nvme_emulated \
> -device nvme,drive=nvme_emulated,serial="test serial",pmrdev=mem1
> In VM:
> My persisent memory region is exposed PCI BAR
> Region 2: Memory at fe000000 (64-bit, prefetchable) [size=1M]
> So I perform reads/writes from/to following adress 0xfe000000 (decimal 
> 4261412864)
> dd if=test.bin of=/dev/mem bs=1 count=30 seek=4261412864
> dd if=/dev/mem of=test1.bin bs=1 count=30 skip=4261412864

Did you verify that the guest kernel is really accessing the BAR?  I
remember that distro kernels often ship with options that make
/dev/mem of limited use because it's considered insecure.

> On VMM I didn't observe that backing file has been updated and after power 
> cycling VM
> I see old junk when reading PMR region.

Did you check that the pmrdev mmap region contains the data the guest
wrote before power cycling?

> Also from include/qemu/pmem.h it looks like pmem_persist() will cause qemu to 
> exit
> if libpmem is not installed:

The libpmem support only needs to be used when the pmem=on option was
given.  If there isn't a physical pmem device then it doesn't need to
be used.


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