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From: | Eric DeVolder |
Subject: | Re: [PATCH v2 3/7] ACPI ERST: support for ACPI ERST feature |
Date: | Tue, 25 May 2021 20:22:26 +0000 |
Igor,
Thank you for the pointers, I've turned the corner on the use of hostmem-file and am able to use it now!
I do have one question regarding hostmem-file. In the VMStateDescription, I used to have this for the contents
of the nvram (backed by a file):
VMSTATE_VARRAY_UINT32(nvram, ERSTDeviceState, prop_size, 0,
vmstate_info_uint8, uint8_t),
I've searched and I do not locate an example of passing a HostMemoryBackend object; how do I do that?
Thanks,
eric
From: Igor Mammedov <imammedo@redhat.com>
Sent: Thursday, May 20, 2021 6:00 AM To: Eric DeVolder <eric.devolder@oracle.com> Cc: ehabkost@redhat.com <ehabkost@redhat.com>; mst@redhat.com <mst@redhat.com>; Konrad Wilk <konrad.wilk@oracle.com>; qemu-devel@nongnu.org <qemu-devel@nongnu.org>; pbonzini@redhat.com <pbonzini@redhat.com>; Boris Ostrovsky <boris.ostrovsky@oracle.com>; rth@twiddle.net <rth@twiddle.net>; jusual@redhat.com <jusual@redhat.com> Subject: Re: [PATCH v2 3/7] ACPI ERST: support for ACPI ERST feature On Tue, 18 May 2021 17:08:31 +0000
Eric DeVolder <eric.devolder@oracle.com> wrote: > Hi Igor, > Thanks for the information. I am primarily interested in ensuring data persistence in the case of #1. > As it stands so far, I have yet to observe any kind of write back into the backing file. Just to summarize, > what I've done thus far is: > > in erst_realizefn(): > ... > s->hostmem_obj = object_new(TYPE_MEMORY_BACKEND_FILE); > object_property_set_str(s->hostmem_obj, "mem-path", (const char *)(TYPE_ACPI_ERST ".hostmem"), &error_fatal); > object_property_set_int(s->hostmem_obj, "size", s->prop_size, &error_fatal); > user_creatable_complete(USER_CREATABLE(s->hostmem_obj), &error_fatal); > s->hostmem = MEMORY_BACKEND(s->hostmem_obj); backend should be provided by user on CLI so all backend's properties are configured there as user desires and frontend should access it via link property. see how pc-dimm's memdev property is used. > and then in erst_update_backing_file(), which is called when records are created/updated: > > ... > if ((mr = host_memory_backend_get_memory(s->hostmem))) { > uint8_t *p = (uint8_t *)memory_region_get_ram_ptr(mr); > memcpy(p + offset, data, length); > memory_region_msync(mr, 0, s->prop_size); /* for now, the whole thing */ > } > > I've instrumented this code, and I can see the records. I've instrumented memory_region_msync() all the way down > to qemu_msync() and it makes it into that code. But the end result has always been the same, the backing file is > never updated. > > I'm not really sure what else I need to do to get the hostmem contents to be written back into the file. see "man mmap" in particular MAP_SHARED vs MAP_PRIVATE and there is a corresponding property for the file backend to manage that. in case #1 no explicit sync is needed, backing file should be updated on close at the latest (whether it's graceful/or forced (i.e. crash)) > Thanks, > eric > > > ________________________________ > From: Igor Mammedov <imammedo@redhat.com> > Sent: Monday, May 17, 2021 11:31 AM > To: Eric DeVolder <eric.devolder@oracle.com> > Cc: ehabkost@redhat.com <ehabkost@redhat.com>; mst@redhat.com <mst@redhat.com>; Konrad Wilk <konrad.wilk@oracle.com>; qemu-devel@nongnu.org <qemu-devel@nongnu.org>; pbonzini@redhat.com <pbonzini@redhat.com>; Boris Ostrovsky <boris.ostrovsky@oracle.com>; rth@twiddle.net <rth@twiddle.net>; jusual@redhat.com <jusual@redhat.com> > Subject: Re: [PATCH v2 3/7] ACPI ERST: support for ACPI ERST feature > > On Mon, 17 May 2021 15:01:02 +0000 > Eric DeVolder <eric.devolder@oracle.com> wrote: > > > Hi Igor, > > I've been working to transition ERST to use the hostmem-file object as the backing store, as requested. > > > > I have the backend-file object now in ERST, and I have a question for you. This hostmem-file initializes > > itself from a file, but in looking at the code, I do not see that it ever writes back to the file!? Furthermore, > > I don't see a "flush" type method to force writeback of data in the object back to file? > > > > The original ERST code would flush/write to the backing file each record as it was created. I don't see > > any equivalent way of doing that with hostmem-file? > > To force flush you can use memory_region_msync() on MemoryRegion that you get from hostmem backend. > But question is what are you trying to achieve with sync > 1. data persistence in case of QEMU crash > 2. data persistence in case of host crash > > for the former you do not need explicit sync as memory buffers should be flushed to disk by kernel > if you put backend on nvdimm, you should get 2 without sync as well (see pmem=on property) > > just do not forget that sync is not free, so if #1 is acceptable I'd avoid explicit sync. > > > > Please point out where I am misunderstanding. > > > > Thanks, > > eric [...] |
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