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Re: [PATCH] hostmem-file: add the 'hmem' option

From: David Hildenbrand
Subject: Re: [PATCH] hostmem-file: add the 'hmem' option
Date: Mon, 9 Dec 2024 22:11:29 +0100
User-agent: Mozilla Thunderbird 
On 06.12.24 18:58, Luo, Zhigang wrote:

[AMD Official Use Only - AMD Internal Distribution Only]

Hi David,


Hi!



Thanks for your comments.
Let me give you some background for this patch.
I am currently engaged in a project that requires to pass the EFI_MEMORY_SP 
(Special Purpose Memory) type memory from host to a virtual machine within 
QEMU. This memory needs to be EFI_MEMORY_SP type in the virtual machine as well.
This particular memory type is essential for the functionality of my project.


Which exact guest memory will be backed by this memory? All guest-memory?

And, what is the guest OS going to do with this memory?
Usually, this SP memory (dax, cxl, ...) is not used as boot memory. Like on a bare metal system, one would expect that only CXL memory will be marked as special and put aside to the cxl driver, such that the OS can boot on ordinary DIMMs, such that cxl can online it etc.
So maybe you would want to expose this memory using CXL-mem device to the VM? Or a DIMM?
I assume the alternative is to tell the VM on the Linux kernel cmdline to set EFI_MEMORY_SP on this memory. I recall that there is a way to achieve that. 


In Linux, the SPM memory will be claimed by hmem-dax driver by default. With 
this patch I can use the following config to pass the SPM memory to guest VM.
-object 
memory-backend-file,size=30G,id=m1,mem-path=/dev/dax0.0,prealloc=on,align=1G,hmem=on

I was thinking to change the option name from "hmem" to "spm" to avoid 
confusion.
Likely it should be specified elsewhere, that you want specific guest RAM ranges to be EFI_MEMORY_SP. For a DIMM, it could be a property, similarly maybe for CXL-mem devices (no expert on that).
For boot memory / machine memory it could be a machine property. But I'll first have to learn which ranges you actually want to expose that way, and what the VM will do with that information. 



Do you have any suggestions to achieve this more reasonable?
The problem with qemu_ram_foreach_block() is that you would indicate also DIMMs, virtio-mem, ... and even RAMBlocks that are not even used for backing anything to the VM as EFI_MEMORY_SP, which is wrong. 



Thanks,
Zhigang

-----Original Message-----
From: David Hildenbrand <david@redhat.com>
Sent: Friday, December 6, 2024 5:08 AM
To: Luo, Zhigang <Zhigang.Luo@amd.com>; qemu-devel@nongnu.org
Cc: kraxel@redhat.com; Igor Mammedov <imammedo@redhat.com>
Subject: Re: [PATCH] hostmem-file: add the 'hmem' option

On 04.12.24 18:11, Zhigang Luo wrote:

This boolean option 'hmem' allows users to set a memory region from
memory-backend-file as heterogeneous memory. If 'hmem=on', QEMU will
set the flag RAM_HMEM in the RAM block of the corresponding memory
region and set the e820 type to E820_SOFT_RESERVED for this region.



Hi,

./scripts/get_maintainer.pl is your friend to figure out whom to CC on patches.

In general: not a fan. You seem to be abusing memory backend properties
+ RAM flags to merely modify how memory is going to be exposed in the
memory map on x86.

It's not even clear why heterogeneous memory should be exposed like
that, and how reasonable it is to essentially expose all of guest RAM as
E820_SOFT_RESERVED.


Note that the whole "pmem=on" case was very different, because it
required mmap() modifications.


Signed-off-by: Zhigang Luo <Zhigang.Luo@amd.com>
---
   backends/hostmem-file.c      | 23 +++++++++++++++++++++++
   hw/i386/e820_memory_layout.h |  1 +
   hw/i386/pc.c                 | 16 ++++++++++++++++
   include/exec/cpu-common.h    |  1 +
   include/exec/memory.h        |  3 +++
   qapi/qom.json                |  4 ++++
   system/physmem.c             |  7 ++++++-
   7 files changed, 54 insertions(+), 1 deletion(-)

diff --git a/backends/hostmem-file.c b/backends/hostmem-file.c
index 7e5072e33e..5ddfdbaf86 100644
--- a/backends/hostmem-file.c
+++ b/backends/hostmem-file.c
@@ -32,6 +32,7 @@ struct HostMemoryBackendFile {
       uint64_t offset;
       bool discard_data;
       bool is_pmem;
+    bool is_hmem;
       bool readonly;
       OnOffAuto rom;
   };
@@ -88,6 +89,7 @@ file_backend_memory_alloc(HostMemoryBackend *backend, Error 
**errp)
       ram_flags |= backend->reserve ? 0 : RAM_NORESERVE;
       ram_flags |= backend->guest_memfd ? RAM_GUEST_MEMFD : 0;
       ram_flags |= fb->is_pmem ? RAM_PMEM : 0;
+    ram_flags |= fb->is_hmem ? RAM_HMEM : 0;
       ram_flags |= RAM_NAMED_FILE;
       return memory_region_init_ram_from_file(&backend->mr, OBJECT(backend), 
name,
                                               backend->size, fb->align, 
ram_flags,
@@ -256,6 +258,25 @@ static void file_memory_backend_set_rom(Object *obj, 
Visitor *v,
       visit_type_OnOffAuto(v, name, &fb->rom, errp);
   }

+static bool file_memory_backend_get_hmem(Object *o, Error **errp)
+{
+    return MEMORY_BACKEND_FILE(o)->is_hmem;
+}
+
+static void file_memory_backend_set_hmem(Object *o, bool value, Error **errp)
+{
+    HostMemoryBackend *backend = MEMORY_BACKEND(o);
+    HostMemoryBackendFile *fb = MEMORY_BACKEND_FILE(o);
+
+    if (host_memory_backend_mr_inited(backend)) {
+        error_setg(errp, "cannot change property 'hmem' of %s.",
+                   object_get_typename(o));
+        return;
+    }
+
+    fb->is_hmem = value;
+}
+
   static void file_backend_unparent(Object *obj)
   {
       HostMemoryBackend *backend = MEMORY_BACKEND(obj);
@@ -295,6 +316,8 @@ file_backend_class_init(ObjectClass *oc, void *data)
       object_class_property_add_bool(oc, "pmem",
           file_memory_backend_get_pmem, file_memory_backend_set_pmem);
   #endif
+    object_class_property_add_bool(oc, "hmem",
+        file_memory_backend_get_hmem, file_memory_backend_set_hmem);
       object_class_property_add_bool(oc, "readonly",
           file_memory_backend_get_readonly,
           file_memory_backend_set_readonly);
diff --git a/hw/i386/e820_memory_layout.h b/hw/i386/e820_memory_layout.h
index b50acfa201..8af6a9cfac 100644
--- a/hw/i386/e820_memory_layout.h
+++ b/hw/i386/e820_memory_layout.h
@@ -15,6 +15,7 @@
   #define E820_ACPI       3
   #define E820_NVS        4
   #define E820_UNUSABLE   5
+#define E820_SOFT_RESERVED  0xEFFFFFFF

   struct e820_entry {
       uint64_t address;
diff --git a/hw/i386/pc.c b/hw/i386/pc.c
index 317aaca25a..41e9cc276c 100644
--- a/hw/i386/pc.c
+++ b/hw/i386/pc.c
@@ -785,6 +785,21 @@ static hwaddr pc_max_used_gpa(PCMachineState *pcms, 
uint64_t pci_hole64_size)
       return pc_above_4g_end(pcms) - 1;
   }

+static int pc_update_hmem_memory(RAMBlock *rb, void *opaque)
+{
+    X86MachineState *x86ms = opaque;
+    ram_addr_t offset;
+    ram_addr_t length;
+
+    if (qemu_ram_is_hmem(rb)) {
+        offset = qemu_ram_get_offset(rb) + (0x100000000ULL - 
x86ms->below_4g_mem_size);
+        length = qemu_ram_get_used_length(rb);
+        e820_add_entry(offset, length, E820_SOFT_RESERVED);
+    }


I am pretty sure this will break in NUMA setups, where we have multiple
memory backends mapped in different locations.

The whole "(0x100000000ULL - x86ms->below_4g_mem_size)" looks hacky.

--
Cheers,

David / dhildenb




--
Cheers,

David / dhildenb
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