On Fri, Oct 10, 2025 at 09:10:08AM +0200, Gerd Hoffmann wrote:
> Starting with the edk2-stable202508 tag OVMF (and ArmVirt too) have
> optional support for logging to a memory buffer. There is guest side
> support -- for example in linux kernels v6.17+ -- to read that buffer.
> But that might not helpful if your guest stops booting early enough that
> guest tooling can not be used yet. So host side support to read that
> log buffer is a useful thing to have.
>
> This patch implements both qmp and hmp monitor commands to read the
> firmware log.
>
> Signed-off-by: Gerd Hoffmann <[email protected]>
> ---
> include/monitor/hmp.h | 1 +
> hw/uefi/ovmf-log.c | 265 +++++++++++++++++++++++++++++++++++++
> tests/qtest/qmp-cmd-test.c | 2 +
> hmp-commands-info.hx | 14 ++
> hw/uefi/meson.build | 2 +-
> qapi/machine.json | 23 ++++
> 6 files changed, 306 insertions(+), 1 deletion(-)
> create mode 100644 hw/uefi/ovmf-log.c
> diff --git a/hw/uefi/ovmf-log.c b/hw/uefi/ovmf-log.c
> new file mode 100644
> index 000000000000..89e27d916531
> --- /dev/null
> +++ b/hw/uefi/ovmf-log.c
> @@ -0,0 +1,265 @@
> +static void handle_ovmf_log_range(GString *out,
> + dma_addr_t start,
> + dma_addr_t end,
> + Error **errp)
> +{
> + g_autofree char *buf = NULL;
> +
> + if (start > end) {
> + return;
> + }
> +
> + buf = g_malloc(end - start + 1);
> + if (dma_memory_read(&address_space_memory, start,
> + buf, end - start,
> + MEMTXATTRS_UNSPECIFIED)) {
> + error_setg(errp, "firmware log: buffer read error");
> + return;
> + }
> +
> + buf[end - start] = 0;
> + g_string_append_printf(out, "%s", buf);
How about eliminating the intermediate buffer alloocation / printf by
reading straight into the GString buf ? Something like
size_t len = end - start;
g_string_set_size(out, out->len + len);
if (dma_memory_read(&address_space_memory, start,
out->str + (out->len - len),
len, MEMTXATTRS_UNSPECIFIED)) {
...
}
With regards,
Daniel
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