Re: [PATCH v7 00/14] KVM: mm: fd-based approach for supporting KVM guest private memory

On 11/08/22 19:02, Chao Peng wrote:

On Thu, Aug 11, 2022 at 01:30:06PM +0200, Gupta, Pankaj wrote:

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While debugging an issue with SEV+UPM, found that fallocate() returns
an error in QEMU which is not handled (EINTR). With the below handling
of EINTR subsequent fallocate() succeeds:

QEMU code has not well-tested so it's not strange you met problem. But
from the man page, there is signal was caught for EINTR, do you know
the signal number?

Thanks for you patch but before we change it in QEMU I want to make sure
it's indeed a QEMU issue (e.g. not a kernel isssue).

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diff --git a/backends/hostmem-memfd-private.c b/backends/hostmem-memfd-private.c
index af8fb0c957..e8597ed28d 100644
--- a/backends/hostmem-memfd-private.c
+++ b/backends/hostmem-memfd-private.c

@@ -39,7 +39,7 @@ priv_memfd_backend_memory_alloc(HostMemoryBackend *backend, Error **errp)
      MachineState *machine = MACHINE(qdev_get_machine());
      uint32_t ram_flags;
      char *name;
-    int fd, priv_fd;
+    int fd, priv_fd, ret;
      if (!backend->size) {
          error_setg(errp, "can't create backend with size 0");

@@ -65,7 +65,15 @@ priv_memfd_backend_memory_alloc(HostMemoryBackend *backend, Error **errp)
                                     backend->size, ram_flags, fd, 0, errp);
      g_free(name);
-    fallocate(priv_fd, 0, 0, backend->size);
+again:
+    ret = fallocate(priv_fd, 0, 0, backend->size);
+    if (ret) {
+           perror("Fallocate failed: \n");
+           if (errno == EINTR)
+                   goto again;
+           else
+                   exit(1);
+    }

However, fallocate() preallocates full guest memory before starting the guest.
With this behaviour guest memory is *not* demand pinned. 

This is with reference to the SEV demand pinning patches that I was working on. 
The understanding was UPM will not reserve memory for SEV/TDX guest in the beginning 
similar to normal guest. Here is the relevant quote from the discussion with Sean[1]:

	"I think we should abandon this approach in favor of committing all our resources
	to fd-based private memory[*], which (if done right) will provide on-demand pinning
	for "free". "

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Is there a way to prevent fallocate() from reserving full guest memory?

Regards
Nikunj
[1] https://lore.kernel.org/kvm/YkIh8zM7XfhsFN8L@google.com/ (local)