KVM: Don't pre-reserve xarray entries when storing empty/NULL attributesgrafted

Skip the xarray reservation loop when clearing all memory attributes, as
storing NULL only erases the entry and never needs to allocate, so no
reservation (and no cleanup of a failed one) is required in that case.

Suggested-by: Sean Christopherson <seanjc@google.com>
Cc: David Ballesteros <davimaba.v@proton.me>
Signed-off-by: Zeng Chi <zengchi@kylinos.cn>
Link: https://patch.msgid.link/20260921102442.1232375-1-zeng_chi911@163.com
[sean: split to separate patch]
Signed-off-by: Sean Christopherson <seanjc@google.com>

1 files changed, 257 insertions, 0 deletions

diff --git a/drivers/scsi/scsicam.c b/drivers/scsi/scsicam.c
new file mode 100644
index 000000000..887de505b
--- /dev/null
+++ b/drivers/scsi/scsicam.c
@@ -0,0 +1,257 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * scsicam.c - SCSI CAM support functions, use for HDIO_GETGEO, etc.
+ *
+ * Copyright 1993, 1994 Drew Eckhardt
+ *      Visionary Computing 
+ *      (Unix and Linux consulting and custom programming)
+ *      drew@Colorado.EDU
+ *      +1 (303) 786-7975
+ *
+ * For more information, please consult the SCSI-CAM draft.
+ */
+
+#include <linux/module.h>
+#include <linux/slab.h>
+#include <linux/fs.h>
+#include <linux/kernel.h>
+#include <linux/blkdev.h>
+#include <linux/pagemap.h>
+#include <linux/msdos_partition.h>
+#include <linux/unaligned.h>
+
+#include <scsi/scsicam.h>
+
+/**
+ * scsi_bios_ptable - Read PC partition table out of first sector of device.
+ * @dev: from this device
+ *
+ * Description: Reads the first sector from the device and returns %0x42 bytes
+ *              starting at offset %0x1be.
+ * Returns: partition table in kmalloc(GFP_KERNEL) memory, or NULL on error.
+ */
+unsigned char *scsi_bios_ptable(struct gendisk *dev)
+{
+	struct address_space *mapping = dev->part0->bd_mapping;
+	unsigned char *res = NULL;
+	struct folio *folio;
+
+	folio = read_mapping_folio(mapping, 0, NULL);
+	if (IS_ERR(folio))
+		return NULL;
+
+	res = kmemdup(folio_address(folio) + 0x1be, 66, GFP_KERNEL);
+	folio_put(folio);
+	return res;
+}
+EXPORT_SYMBOL(scsi_bios_ptable);
+
+/**
+ * scsi_partsize - Parse cylinders/heads/sectors from PC partition table
+ * @disk: gendisk of the disk to parse
+ * @capacity: size of the disk in sectors
+ * @geom: output in form of [hds, cylinders, sectors]
+ *
+ * Determine the BIOS mapping/geometry used to create the partition
+ * table, storing the results in @geom.
+ *
+ * Returns: %false on failure, %true on success.
+ */
+bool scsi_partsize(struct gendisk *disk, sector_t capacity, int geom[3])
+{
+	int cyl, ext_cyl, end_head, end_cyl, end_sector;
+	unsigned int logical_end, physical_end, ext_physical_end;
+	struct msdos_partition *p, *largest = NULL;
+	void *buf;
+	int ret = false;
+
+	buf = scsi_bios_ptable(disk);
+	if (!buf)
+		return false;
+
+	if (*(unsigned short *) (buf + 64) == 0xAA55) {
+		int largest_cyl = -1, i;
+
+		for (i = 0, p = buf; i < 4; i++, p++) {
+			if (!p->sys_ind)
+				continue;
+#ifdef DEBUG
+			printk("scsicam_bios_param : partition %d has system \n",
+			       i);
+#endif
+			cyl = p->cyl + ((p->sector & 0xc0) << 2);
+			if (cyl > largest_cyl) {
+				largest_cyl = cyl;
+				largest = p;
+			}
+		}
+	}
+	if (largest) {
+		end_cyl = largest->end_cyl + ((largest->end_sector & 0xc0) << 2);
+		end_head = largest->end_head;
+		end_sector = largest->end_sector & 0x3f;
+
+		if (end_head + 1 == 0 || end_sector == 0)
+			goto out_free_buf;
+
+#ifdef DEBUG
+		printk("scsicam_bios_param : end at h = %d, c = %d, s = %d\n",
+		       end_head, end_cyl, end_sector);
+#endif
+
+		physical_end = end_cyl * (end_head + 1) * end_sector +
+		    end_head * end_sector + end_sector;
+
+		/* This is the actual _sector_ number at the end */
+		logical_end = get_unaligned_le32(&largest->start_sect)
+		    + get_unaligned_le32(&largest->nr_sects);
+
+		/* This is for >1023 cylinders */
+		ext_cyl = (logical_end - (end_head * end_sector + end_sector))
+		    / (end_head + 1) / end_sector;
+		ext_physical_end = ext_cyl * (end_head + 1) * end_sector +
+		    end_head * end_sector + end_sector;
+
+#ifdef DEBUG
+		printk("scsicam_bios_param : logical_end=%d physical_end=%d ext_physical_end=%d ext_cyl=%d\n"
+		  ,logical_end, physical_end, ext_physical_end, ext_cyl);
+#endif
+
+		if (logical_end == physical_end ||
+		    (end_cyl == 1023 && ext_physical_end == logical_end)) {
+			geom[0] = end_head + 1;
+			geom[1] = end_sector;
+			geom[2] = (unsigned long)capacity /
+				((end_head + 1) * end_sector);
+			ret = true;
+			goto out_free_buf;
+		}
+#ifdef DEBUG
+		printk("scsicam_bios_param : logical (%u) != physical (%u)\n",
+		       logical_end, physical_end);
+#endif
+	}
+
+out_free_buf:
+	kfree(buf);
+	return ret;
+}
+EXPORT_SYMBOL(scsi_partsize);
+
+/*
+ * Function : static int setsize(unsigned long capacity,unsigned int *cyls,
+ *      unsigned int *hds, unsigned int *secs);
+ *
+ * Purpose : to determine a near-optimal int 0x13 mapping for a
+ *      SCSI disk in terms of lost space of size capacity, storing
+ *      the results in *cyls, *hds, and *secs.
+ *
+ * Returns : -1 on failure, 0 on success.
+ *
+ * Extracted from
+ *
+ * WORKING                                                    X3T9.2
+ * DRAFT                                                        792D
+ * see http://www.t10.org/ftp/t10/drafts/cam/cam-r12b.pdf
+ *
+ *                                                        Revision 6
+ *                                                         10-MAR-94
+ * Information technology -
+ * SCSI-2 Common access method
+ * transport and SCSI interface module
+ * 
+ * ANNEX A :
+ *
+ * setsize() converts a read capacity value to int 13h
+ * head-cylinder-sector requirements. It minimizes the value for
+ * number of heads and maximizes the number of cylinders. This
+ * will support rather large disks before the number of heads
+ * will not fit in 4 bits (or 6 bits). This algorithm also
+ * minimizes the number of sectors that will be unused at the end
+ * of the disk while allowing for very large disks to be
+ * accommodated. This algorithm does not use physical geometry. 
+ */
+
+static int setsize(unsigned long capacity, unsigned int *cyls, unsigned int *hds,
+		   unsigned int *secs)
+{
+	unsigned int rv = 0;
+	unsigned long heads, sectors, cylinders, temp;
+
+	cylinders = 1024L;	/* Set number of cylinders to max */
+	sectors = 62L;		/* Maximize sectors per track */
+
+	temp = cylinders * sectors;	/* Compute divisor for heads */
+	heads = capacity / temp;	/* Compute value for number of heads */
+	if (capacity % temp) {	/* If no remainder, done! */
+		heads++;	/* Else, increment number of heads */
+		temp = cylinders * heads;	/* Compute divisor for sectors */
+		sectors = capacity / temp;	/* Compute value for sectors per
+						   track */
+		if (capacity % temp) {	/* If no remainder, done! */
+			sectors++;	/* Else, increment number of sectors */
+			temp = heads * sectors;		/* Compute divisor for cylinders */
+			cylinders = capacity / temp;	/* Compute number of cylinders */
+		}
+	}
+	if (cylinders == 0)
+		rv = (unsigned) -1;	/* Give error if 0 cylinders */
+
+	*cyls = (unsigned int) cylinders;	/* Stuff return values */
+	*secs = (unsigned int) sectors;
+	*hds = (unsigned int) heads;
+	return (rv);
+}
+
+/**
+ * scsicam_bios_param - Determine geometry of a disk in cylinders/heads/sectors.
+ * @disk: which device
+ * @capacity: size of the disk in sectors
+ * @ip: return value: ip[0]=heads, ip[1]=sectors, ip[2]=cylinders
+ *
+ * Description : determine the BIOS mapping/geometry used for a drive in a
+ *      SCSI-CAM system, storing the results in ip as required
+ *      by the HDIO_GETGEO ioctl().
+ *
+ * Returns : -1 on failure, 0 on success.
+ */
+int scsicam_bios_param(struct gendisk *disk, sector_t capacity, int *ip)
+{
+	u64 capacity64 = capacity;	/* Suppress gcc warning */
+	int ret = 0;
+
+	/* try to infer mapping from partition table */
+	if (scsi_partsize(disk, capacity, ip))
+		return 0;
+
+	if (capacity64 < (1ULL << 32)) {
+		/*
+		 * Pick some standard mapping with at most 1024 cylinders, and
+		 * at most 62 sectors per track - this works up to 7905 MB.
+		 */
+		ret = setsize((unsigned long)capacity, (unsigned int *)ip + 2,
+		       (unsigned int *)ip + 0, (unsigned int *)ip + 1);
+	}
+
+	/*
+	 * If something went wrong, then apparently we have to return a geometry
+	 * with more than 1024 cylinders.
+	 */
+	if (ret || ip[0] > 255 || ip[1] > 63) {
+		if ((capacity >> 11) > 65534) {
+			ip[0] = 255;
+			ip[1] = 63;
+		} else {
+			ip[0] = 64;
+			ip[1] = 32;
+		}
+
+		if (capacity > 65535*63*255)
+			ip[2] = 65535;
+		else
+			ip[2] = (unsigned long)capacity / (ip[0] * ip[1]);
+	}
+
+	return 0;
+}
+EXPORT_SYMBOL(scsicam_bios_param);