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From: Stefan Hajnoczi <stefanha@redhat.com>
To: qemu-devel@nongnu.org, Eduardo Habkost <ehabkost@redhat.com>,
	Igor Mammedov <imammedo@redhat.com>,
	Paolo Bonzini <pbonzini@redhat.com>,
	mst@redhat.com, Xiao Guangrong <xiaoguangrong.eric@gmail.com>,
	Juan Quintela <quintela@redhat.com>,
	dgilbert@redhat.com, Dan Williams <dan.j.williams@intel.com>
Subject: Re: [Qemu-devel] [PATCH v2 4/8] mem/nvdimm: ensure write persistence to PMEM in label emulation
Date: Mon, 12 Feb 2018 13:55:40 +0000	[thread overview]
Message-ID: <20180212135540.GE4913@stefanha-x1.localdomain> (raw)
In-Reply-To: <20180209145726.55ushghorccxne47@hz-desktop>

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On Fri, Feb 09, 2018 at 10:57:26PM +0800, Haozhong Zhang wrote:
> On 02/09/18 14:27 +0000, Stefan Hajnoczi wrote:
> > On Wed, Feb 07, 2018 at 03:33:27PM +0800, Haozhong Zhang wrote:
> > > @@ -156,11 +157,17 @@ static void nvdimm_write_label_data(NVDIMMDevice *nvdimm, const void *buf,
> > >  {
> > >      MemoryRegion *mr;
> > >      PCDIMMDevice *dimm = PC_DIMM(nvdimm);
> > > +    bool is_pmem = object_property_get_bool(OBJECT(dimm->hostmem),
> > > +                                            "pmem", NULL);
> > >      uint64_t backend_offset;
> > >  
> > >      nvdimm_validate_rw_label_data(nvdimm, size, offset);
> > >  
> > > -    memcpy(nvdimm->label_data + offset, buf, size);
> > > +    if (!is_pmem) {
> > > +        memcpy(nvdimm->label_data + offset, buf, size);
> > > +    } else {
> > > +        pmem_memcpy_persist(nvdimm->label_data + offset, buf, size);
> > > +    }
> > 
> > Is this enough to prevent label corruption in case of power failure?
> > 
> > pmem_memcpy_persist() is not atomic.  Power failure can result in a mix
> > of the old and new label data.
> > 
> > If we want this operation to be 100% safe there needs to be some kind of
> > update protocol that makes the change atomic, like a Label A and Label B
> > area with a single Label Index field that can be updated atomically to
> > point to the active Label A/B area.
> 
> All this patch series is to guarantee: if the guest is still alive and
> running, all its previous writes to pmem, which were performed by
> QEMU, will be still persistent on pmem.
> 
> If a power failure happens before QEMU returns to the guest, e.g., in
> the middle of above pmem_memcpy_persist(), yes, the guest label data
> may be in an inconsistent state, but the guest also has no chance to
> progress.  And, that is what could happen in the non-virtualization
> environment as well, and it's the responsibility of the (guest) SW to
> defend such failures, e.g., by the protocol you mentioned.

Thanks for explaining!  I thought the atomic update is done by the ACPI
_LSW DSM method but it turns out the driver must do it.

For anyone else who is interested, a detailed description of the Label
Storage Area is on page 652 of the UEFI Specification version 2.7 and
the Linux driver implements the atomic update algorithm in
drivers/nvdimm/label.c:__pmem_label_update().

Stefan

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  reply	other threads:[~2018-02-12 13:56 UTC|newest]

Thread overview: 34+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-02-07  7:33 [Qemu-devel] [PATCH v2 0/8] nvdimm: guarantee persistence of QEMU writes to persistent memory Haozhong Zhang
2018-02-07  7:33 ` [Qemu-devel] [PATCH v2 1/8] memory, exec: switch file ram allocation functions to 'flags' parameters Haozhong Zhang
2018-02-07  7:33 ` [Qemu-devel] [PATCH v2 2/8] hostmem-file: add the 'pmem' option Haozhong Zhang
2018-02-07  7:33 ` [Qemu-devel] [PATCH v2 3/8] configure: add libpmem support Haozhong Zhang
2018-02-07  7:33 ` [Qemu-devel] [PATCH v2 4/8] mem/nvdimm: ensure write persistence to PMEM in label emulation Haozhong Zhang
2018-02-09 14:27   ` Stefan Hajnoczi
2018-02-09 14:57     ` Haozhong Zhang
2018-02-12 13:55       ` Stefan Hajnoczi [this message]
2018-02-07  7:33 ` [Qemu-devel] [PATCH v2 5/8] migration/ram: ensure write persistence on loading zero pages to PMEM Haozhong Zhang
2018-02-07 10:17   ` Pankaj Gupta
2018-02-07 11:18     ` Haozhong Zhang
2018-02-07 11:30       ` Pankaj Gupta
2018-02-07 11:38   ` Dr. David Alan Gilbert
2018-02-07 11:52     ` Haozhong Zhang
2018-02-07 12:51       ` Haozhong Zhang
2018-02-07 12:59         ` Dr. David Alan Gilbert
2018-02-07 14:10         ` Pankaj Gupta
2018-02-07 12:56       ` Dr. David Alan Gilbert
2018-02-07  7:33 ` [Qemu-devel] [PATCH v2 6/8] migration/ram: ensure write persistence on loading normal " Haozhong Zhang
2018-02-07 11:49   ` Dr. David Alan Gilbert
2018-02-07 12:02     ` Haozhong Zhang
2018-02-07  7:33 ` [Qemu-devel] [PATCH v2 7/8] migration/ram: ensure write persistence on loading compressed " Haozhong Zhang
2018-02-07 11:54   ` Dr. David Alan Gilbert
2018-02-07 12:15     ` Haozhong Zhang
2018-02-07 13:03       ` Dr. David Alan Gilbert
2018-02-07 13:20         ` Haozhong Zhang
2018-02-07 13:24           ` Dr. David Alan Gilbert
2018-02-07 18:05             ` Dan Williams
2018-02-07 18:08               ` Dr. David Alan Gilbert
2018-02-07 18:31                 ` Dan Williams
2018-02-07 18:37                   ` Dr. David Alan Gilbert
2018-02-07 22:43                     ` Dan Williams
2018-02-07  7:33 ` [Qemu-devel] [PATCH v2 8/8] migration/ram: ensure write persistence on loading xbzrle " Haozhong Zhang
2018-02-07 13:08   ` Dr. David Alan Gilbert

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