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Re: [Qemu-block] [Qemu-devel] [RFC PATCH] hw/arm/virt: use variable size


From: Xiang Zheng
Subject: Re: [Qemu-block] [Qemu-devel] [RFC PATCH] hw/arm/virt: use variable size of flash device to save memory
Date: Fri, 12 Apr 2019 17:50:09 +0800
User-agent: Mozilla/5.0 (Windows NT 6.1; WOW64; rv:64.0) Gecko/20100101 Thunderbird/64.0

On 2019/4/12 9:52, Xiang Zheng wrote:
> On 2019/4/11 20:22, Kevin Wolf wrote:
>> Okay, so your problem is that blk_pread() writes to the whole buffer,
>> writing explicit zeroes for unallocated parts of the image, while you
>> would like to leave those parts of the buffer untouched so that we don't
>> actually allocate the memory, but can just use the shared zero page.
>>
>> If you just want to read the non-zero parts of the image, that can be
>> done by using a loop that calls bdrv_block_status() and only reads from
>> the image if the BDRV_BLOCK_ZERO bit is clear.
>>
>> Would this solve your problem?
> 
> Sounds good! What if guest tried to read/write the zero parts?
> 

I wrote the below patch (refer to bdrv_make_zero()) for test, it seems
that everything is OK and the memory is also exactly allocated on demand.

This requires pflash devices to use sparse files backend. Thus I have to
create images like:

   dd of="QEMU_EFI-pflash.raw" if="/dev/zero" bs=1M seek=64 count=0
   dd of="QEMU_EFI-pflash.raw" if="QEMU_EFI.fd" conv=notrunc

   dd of="empty_VARS.fd" if="/dev/zero" bs=1M seek=64 count=0


---8>---

diff --git a/block/block-backend.c b/block/block-backend.c
index f78e82a..ed8ca87 100644
--- a/block/block-backend.c
+++ b/block/block-backend.c
@@ -1379,6 +1379,12 @@ BlockAIOCB *blk_aio_pwrite_zeroes(BlockBackend *blk, 
int64_t offset,
                         flags | BDRV_REQ_ZERO_WRITE, cb, opaque);
 }

+int blk_pread_nonzeroes(BlockBackend *blk, void *buf)
+{
+    int ret = bdrv_pread_nonzeroes(blk->root, buf);
+    return ret;
+}
+
 int blk_pread(BlockBackend *blk, int64_t offset, void *buf, int count)
 {
     int ret = blk_prw(blk, offset, buf, count, blk_read_entry, 0);
diff --git a/block/io.c b/block/io.c
index dfc153b..83e5ea7 100644
--- a/block/io.c
+++ b/block/io.c
@@ -882,6 +882,38 @@ int bdrv_pwrite_zeroes(BdrvChild *child, int64_t offset,
                         BDRV_REQ_ZERO_WRITE | flags);
 }

+int bdrv_pread_nonzeroes(BdrvChild *child, void *buf)
+{
+    int ret;
+    int64_t target_size, bytes, offset = 0;
+    BlockDriverState *bs = child->bs;
+
+    target_size = bdrv_getlength(bs);
+    if (target_size < 0) {
+        return target_size;
+    }
+
+    for (;;) {
+        bytes = MIN(target_size - offset, BDRV_REQUEST_MAX_BYTES);
+        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) {
+            offset += bytes;
+            continue;
+        }
+        ret = bdrv_pread(child, offset, buf, bytes);
+        if (ret < 0) {
+            return ret;
+        }
+        offset += bytes;
+    }
+}
+
 /*
  * Completely zero out a block device with the help of bdrv_pwrite_zeroes.
  * The operation is sped up by checking the block status and only writing
diff --git a/hw/block/block.c b/hw/block/block.c
index bf56c76..e3c67f8 100644
--- a/hw/block/block.c
+++ b/hw/block/block.c
@@ -53,7 +53,7 @@ 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, buf, size);
+    ret = blk_pread_nonzeroes(blk, buf);
     if (ret < 0) {
         error_setg_errno(errp, -ret, "can't read block backend");
         return false;
diff --git a/include/block/block.h b/include/block/block.h
index c7a2619..d0e06cf 100644
--- a/include/block/block.h
+++ b/include/block/block.h
@@ -322,6 +322,7 @@ int bdrv_write(BdrvChild *child, int64_t sector_num,
                const uint8_t *buf, int nb_sectors);
 int bdrv_pwrite_zeroes(BdrvChild *child, int64_t offset,
                        int bytes, BdrvRequestFlags flags);
+int bdrv_pread_nonzeroes(BdrvChild *child, void *buf);
 int bdrv_make_zero(BdrvChild *child, BdrvRequestFlags flags);
 int bdrv_pread(BdrvChild *child, int64_t offset, void *buf, int bytes);
 int bdrv_preadv(BdrvChild *child, int64_t offset, QEMUIOVector *qiov);
diff --git a/include/sysemu/block-backend.h b/include/sysemu/block-backend.h
index 3be05c2..5d349d2 100644
--- a/include/sysemu/block-backend.h
+++ b/include/sysemu/block-backend.h
@@ -129,6 +129,7 @@ int blk_pwrite_zeroes(BlockBackend *blk, int64_t offset,
 BlockAIOCB *blk_aio_pwrite_zeroes(BlockBackend *blk, int64_t offset,
                                   int bytes, BdrvRequestFlags flags,
                                   BlockCompletionFunc *cb, void *opaque);
+int blk_pread_nonzeroes(BlockBackend *blk, void *buf);
 int blk_make_zero(BlockBackend *blk, BdrvRequestFlags flags);
 int blk_pread(BlockBackend *blk, int64_t offset, void *buf, int bytes);
 int blk_pwrite(BlockBackend *blk, int64_t offset, const void *buf, int bytes,
-- 
1.8.3.1



-- 

Thanks,
Xiang





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