On 21.01.25 18:39, Peter Xu wrote:
On Wed, Mar 20, 2024 at 03:39:03AM -0500, Michael Roth wrote:
From: Xiaoyao Li <xiaoyao.li@xxxxxxxxx>
Add a new member "guest_memfd" to memory backends. When it's set
to true, it enables RAM_GUEST_MEMFD in ram_flags, thus private kvm
guest_memfd will be allocated during RAMBlock allocation.
Memory backend's @guest_memfd is wired with @require_guest_memfd
field of MachineState. It avoid looking up the machine in phymem.c.
MachineState::require_guest_memfd is supposed to be set by any VMs
that requires KVM guest memfd as private memory, e.g., TDX VM.
Signed-off-by: Xiaoyao Li <xiaoyao.li@xxxxxxxxx>
Reviewed-by: David Hildenbrand <david@xxxxxxxxxx>
---
Changes in v4:
- rename "require_guest_memfd" to "guest_memfd" in struct
HostMemoryBackend; (David Hildenbrand)
Signed-off-by: Michael Roth <michael.roth@xxxxxxx>
---
backends/hostmem-file.c | 1 +
backends/hostmem-memfd.c | 1 +
backends/hostmem-ram.c | 1 +
backends/hostmem.c | 1 +
hw/core/machine.c | 5 +++++
include/hw/boards.h | 2 ++
include/sysemu/hostmem.h | 1 +
7 files changed, 12 insertions(+)
diff --git a/backends/hostmem-file.c b/backends/hostmem-file.c
index ac3e433cbd..3c69db7946 100644
--- a/backends/hostmem-file.c
+++ b/backends/hostmem-file.c
@@ -85,6 +85,7 @@ file_backend_memory_alloc(HostMemoryBackend *backend, Error **errp)
ram_flags |= fb->readonly ? RAM_READONLY_FD : 0;
ram_flags |= fb->rom == ON_OFF_AUTO_ON ? RAM_READONLY : 0;
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 |= RAM_NAMED_FILE;
return memory_region_init_ram_from_file(&backend->mr, OBJECT(backend), name,
diff --git a/backends/hostmem-memfd.c b/backends/hostmem-memfd.c
index 3923ea9364..745ead0034 100644
--- a/backends/hostmem-memfd.c
+++ b/backends/hostmem-memfd.c
@@ -55,6 +55,7 @@ memfd_backend_memory_alloc(HostMemoryBackend *backend, Error **errp)
name = host_memory_backend_get_name(backend);
ram_flags = backend->share ? RAM_SHARED : 0;
ram_flags |= backend->reserve ? 0 : RAM_NORESERVE;
+ ram_flags |= backend->guest_memfd ? RAM_GUEST_MEMFD : 0;
return memory_region_init_ram_from_fd(&backend->mr, OBJECT(backend), name,
backend->size, ram_flags, fd, 0, errp);
}
diff --git a/backends/hostmem-ram.c b/backends/hostmem-ram.c
index d121249f0f..f7d81af783 100644
--- a/backends/hostmem-ram.c
+++ b/backends/hostmem-ram.c
@@ -30,6 +30,7 @@ ram_backend_memory_alloc(HostMemoryBackend *backend, Error **errp)
name = host_memory_backend_get_name(backend);
ram_flags = backend->share ? RAM_SHARED : 0;
ram_flags |= backend->reserve ? 0 : RAM_NORESERVE;
+ ram_flags |= backend->guest_memfd ? RAM_GUEST_MEMFD : 0;
return memory_region_init_ram_flags_nomigrate(&backend->mr, OBJECT(backend),
name, backend->size,
ram_flags, errp);
These change look a bit confusing to me, as I don't see how gmemfd can be
used with either file or ram typed memory backends..
I recall that the following should work:
"private" memory will come from guest_memfd, "shared" (as in, accessible
by the host) will come from anonymous memory.
This "anon" memory cannot be "shared" with other processes, but
virtio-kernel etc. can just use it.
To "share" the memory with other processes, we'd need memfd/file.
When specified gmemfd=on with those, IIUC it'll allocate both the memory
(ramblock->host) and gmemfd, but without using ->host. Meanwhile AFAIU the
ramblock->host will start to conflict with gmemfd in the future when it
might be able to be mapp-able (having valid ->host).
These will require a new guest_memfd memory backend (I recall that was
discussed a couple of times).
I have a local fix for this (and actually more than below.. but starting
from it), I'm not sure whether I overlooked something, but from reading the
cover letter it's only using memfd backend which makes perfect sense to me
so far.
Does the anon+guest_memfd combination not work or are you speculating
about the usability (which I hopefully addressed above).
--
Cheers,
David / dhildenb
