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

[Andrzej Jakowski <andrzej.jakowski@linux.intel.com>]

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)) \
-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

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.

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

#ifndef QEMU_PMEM_H
#define QEMU_PMEM_H

#ifdef CONFIG_LIBPMEM
#include <libpmem.h>
#else  /* !CONFIG_LIBPMEM */

static inline void *
pmem_memcpy_persist(void *pmemdest, const void *src, size_t len)
{
    /* If 'pmem' option is 'on', we should always have libpmem support,
       or qemu will report a error and exit, never come here. */
    g_assert_not_reached();
    return NULL;
}

static inline void
pmem_persist(const void *addr, size_t len)
{
    g_assert_not_reached();
}

#endif /* CONFIG_LIBPMEM */

#endif /* QEMU_PMEM_H */