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[Qemu-devel] Re: [PATCH 1/4] Add a scatter-gather list type and accessor

From: Avi Kivity
Subject: [Qemu-devel] Re: [PATCH 1/4] Add a scatter-gather list type and accessors
Date: Wed, 04 Feb 2009 22:30:05 +0200
User-agent: Thunderbird (X11/20090105)

Anthony Liguori wrote:
Avi Kivity wrote:
Scatter-gather lists are used extensively in dma-capable devices; a
single data structure allows more code reuse later on.

diff --git a/dma-helpers.c b/dma-helpers.c
new file mode 100644
index 0000000..315834e
--- /dev/null
+++ b/dma-helpers.c
@@ -0,0 +1,29 @@

Needs copyright/license.


Is it possible to have a blanket license for files which don't have explicit terms? I don't much like boilerplate.

+#include "dma.h"
+void qemu_sglist_init(QEMUSGList *qsg, int alloc_hint)
+    qsg->sg = qemu_malloc(alloc_hint * sizeof(ScatterGatherEntry));

Would be nice to check for malloc failures and fail gracefully at least.

Do you mean an exit(1)?  If so we could just put it in qemu_malloc().

Propagation is usually not possible since hardware is not expected to run out of memory. It will also be a never-tested code path.

+void qemu_sglist_add(QEMUSGList *qsg, target_phys_addr_t base,
+                     target_phys_addr_t len)
+    if (qsg->nsg == qsg->nalloc) {
+        qsg->nalloc = 2 * qsg->nalloc + 1;

Do you really want exponential growth verses linear growth? The sg lists should be relatively small so linear growth should be fine.

I expect this to trigger rarely since the allocation hint should suffice nearly 100% of the time. But in case we miss, it's better to reallocate as little as possible.

(what I really want is std::vector<>)

I have a truly marvellous patch that fixes the bug which this
signature is too narrow to contain.

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