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pflash: Only read non-zero parts of backend image

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Currently we fill the VIRT_FLASH memory space with two 64MB NOR images
when using persistent UEFI variables on virt board. Actually we only use
a very small(non-zero) part of the memory while the rest significant
large(zero) part of memory is wasted.

So this patch checks the block status and only writes the non-zero part
into memory. This requires pflash devices to use sparse files for
backends.

Signed-off-by: Xiang Zheng <zhengxiang9@huawei.com>

[ kraxel: rebased to latest master ]

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Message-Id: <20221220084246.1984871-1-kraxel@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
This commit is contained in:
Xiang Zheng 2022-12-20 09:42:46 +01:00 committed by Kevin Wolf
parent 95988739c7
commit a4b15a8b9e
1 changed files with 35 additions and 1 deletions
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36
hw/block/block.c
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@ -15,6 +15,40 @@
#include "qapi/error.h"
#include "qapi/qapi-types-block.h"
/*
* Read the non-zeroes parts of @blk into @buf
* Reading all of the @blk is expensive if the zeroes parts of @blk
* is large enough. Therefore check the block status and only write
* the non-zeroes block into @buf.
*
* Return 0 on success, non-zero on error.
*/
static int blk_pread_nonzeroes(BlockBackend *blk, hwaddr size, void *buf)
{
int ret;
int64_t bytes, offset = 0;
BlockDriverState *bs = blk_bs(blk);
for (;;) {
bytes = MIN(size - offset, BDRV_REQUEST_MAX_SECTORS);
if (bytes <= 0) {
return 0;
}
ret = bdrv_block_status(bs, offset, bytes, &bytes, NULL, NULL);
if (ret < 0) {
return ret;
}
if (!(ret & BDRV_BLOCK_ZERO)) {
ret = bdrv_pread(bs->file, offset, bytes,
(uint8_t *) buf + offset, 0);
if (ret < 0) {
return ret;
}
}
offset += bytes;
}
}
/*
* Read the entire contents of @blk into @buf.
* @blk's contents must be @size bytes, and @size must be at most
@ -54,7 +88,7 @@ bool blk_check_size_and_read_all(BlockBackend *blk, void *buf, hwaddr size,
* block device and read only on demand.
*/
assert(size <= BDRV_REQUEST_MAX_BYTES);
ret = blk_pread(blk, 0, size, buf, 0);
ret = blk_pread_nonzeroes(blk, size, buf);
if (ret < 0) {
error_setg_errno(errp, -ret, "can't read block backend");
return false;