Merge tag 'hyperv-fixes-signed-20240303' of git://git.kernel.org/pub/scm/linux/kernel/git/hyperv/linux

Pull hyperv fixes from Wei Liu:

 - Multiple fixes, cleanups and documentations for Hyper-V core code and
   drivers

* tag 'hyperv-fixes-signed-20240303' of git://git.kernel.org/pub/scm/linux/kernel/git/hyperv/linux:
  Drivers: hv: vmbus: make hv_bus const
  x86/hyperv: Allow 15-bit APIC IDs for VTL platforms
  x86/hyperv: Make encrypted/decrypted changes safe for load_unaligned_zeropad()
  x86/mm: Regularize set_memory_p() parameters and make non-static
  x86/hyperv: Use slow_virt_to_phys() in page transition hypervisor callback
  Documentation: hyperv: Add overview of PCI pass-thru device support
  Drivers: hv: vmbus: Update indentation in create_gpadl_header()
  Drivers: hv: vmbus: Remove duplication and cleanup code in create_gpadl_header()
  fbdev/hyperv_fb: Fix logic error for Gen2 VMs in hvfb_getmem()
  Drivers: hv: vmbus: Calculate ring buffer size for more efficient use of memory
  hv_utils: Allow implicit ICTIMESYNCFLAG_SYNC

1 files changed, 21 insertions, 1 deletions

diff --git a/include/linux/hyperv.h b/include/linux/hyperv.h
index 2b00faf98017..6ef0557b4bff 100644
--- a/include/linux/hyperv.h
+++ b/include/linux/hyperv.h
@@ -164,8 +164,28 @@ struct hv_ring_buffer {
 	u8 buffer[];
 } __packed;
 
+
+/*
+ * If the requested ring buffer size is at least 8 times the size of the
+ * header, steal space from the ring buffer for the header. Otherwise, add
+ * space for the header so that is doesn't take too much of the ring buffer
+ * space.
+ *
+ * The factor of 8 is somewhat arbitrary. The goal is to prevent adding a
+ * relatively small header (4 Kbytes on x86) to a large-ish power-of-2 ring
+ * buffer size (such as 128 Kbytes) and so end up making a nearly twice as
+ * large allocation that will be almost half wasted. As a contrasting example,
+ * on ARM64 with 64 Kbyte page size, we don't want to take 64 Kbytes for the
+ * header from a 128 Kbyte allocation, leaving only 64 Kbytes for the ring.
+ * In this latter case, we must add 64 Kbytes for the header and not worry
+ * about what's wasted.
+ */
+#define VMBUS_HEADER_ADJ(payload_sz) \
+	((payload_sz) >=  8 * sizeof(struct hv_ring_buffer) ? \
+	0 : sizeof(struct hv_ring_buffer))
+
 /* Calculate the proper size of a ringbuffer, it must be page-aligned */
-#define VMBUS_RING_SIZE(payload_sz) PAGE_ALIGN(sizeof(struct hv_ring_buffer) + \
+#define VMBUS_RING_SIZE(payload_sz) PAGE_ALIGN(VMBUS_HEADER_ADJ(payload_sz) + \
 					       (payload_sz))
 
 struct hv_ring_buffer_info {