On 28.02.25 18:22, Fuad Tabba wrote:
Hi Peter,
On Fri, 28 Feb 2025 at 08:24, Peter Xu <peterx@xxxxxxxxxx> wrote:
On Tue, Feb 18, 2025 at 05:24:54PM +0000, Fuad Tabba wrote:
Add the KVM capability KVM_CAP_GMEM_SHARED_MEM, which indicates
that the VM supports shared memory in guest_memfd, or that the
host can create VMs that support shared memory. Supporting shared
memory implies that memory can be mapped when shared with the
host.
Signed-off-by: Fuad Tabba <tabba@xxxxxxxxxx>
---
include/uapi/linux/kvm.h | 1 +
virt/kvm/kvm_main.c | 4 ++++
2 files changed, 5 insertions(+)
diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h
index 45e6d8fca9b9..117937a895da 100644
--- a/include/uapi/linux/kvm.h
+++ b/include/uapi/linux/kvm.h
@@ -929,6 +929,7 @@ struct kvm_enable_cap {
#define KVM_CAP_PRE_FAULT_MEMORY 236
#define KVM_CAP_X86_APIC_BUS_CYCLES_NS 237
#define KVM_CAP_X86_GUEST_MODE 238
+#define KVM_CAP_GMEM_SHARED_MEM 239
I think SHARED_MEM is ok. Said that, to me the use case in this series is
more about "in-place" rather than "shared".
In comparison, what I'm recently looking at is a "more" shared mode of
guest-memfd where it works almost like memfd. So all pages will be shared
there.
That helps me e.g. for the N:1 kvm binding issue I mentioned in another
email (in one of my relies in previous version), in which case I want to
enable gmemfd folios to be mapped more than once in a process.
That'll work there as long as it's fully shared, because all things can be
registered in the old VA way, then there's no need to have N:1 restriction.
IOW, gmemfd will still rely on mmu notifier for tearing downs, and the
gmem->bindings will always be empty.
So if this one would be called "in-place", then I'll have my use case as
"shared".
I understand what you mean. The naming here is to be consistent with
the rest of the series. I don't really have a strong opinion. It means
SHARED_IN_PLACE, but then that would be a mouthful. :)
I'll note that Patrick is also driving it in "all shared" mode for his
direct-map removal series IIRC.
So we would have
a) All private
b) Mixing of private and shared (incl conversion)
c) All shared
"IN_PLACE" might be the wrong angle to look at it.
--
Cheers,
David / dhildenb
