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Date: Tue, 27 Feb 2018 02:04:49 +0200
From: "Michael S. Tsirkin" <mst@...hat.com>
To: Marc-André Lureau <marcandre.lureau@...hat.com>
Cc: linux-kernel@...r.kernel.org, slp@...hat.com, bhe@...hat.com,
somlo@....edu, xiaolong.ye@...el.com
Subject: Re: [PATCH v15 11/11] RFC: fw_cfg: do DMA read operation
On Thu, Feb 15, 2018 at 10:33:12PM +0100, Marc-André Lureau wrote:
> Modify fw_cfg_read_blob() to use DMA if the device supports it.
> Return errors, because the operation may fail.
>
> So far, only one call in fw_cfg_register_dir_entries() is using
> kmalloc'ed buf and is thus clearly eligible to DMA read.
>
> Initially, I didn't implement DMA read to speed up boot time, but as a
> first step before introducing DMA write (since read operations were
> already presents). Even more, I didn't realize fw-cfg entries were
> being read by the kernel during boot by default. But actally fw-cfg
> entries are being populated during module probe. I knew DMA improved a
> lot bios boot time (the main reason the DMA interface was added
> afaik). Let see the time it would take to read the whole ACPI
> tables (128kb allocated)
>
> # time cat /sys/firmware/qemu_fw_cfg/by_name/etc/acpi/tables/raw
> - with DMA: sys 0m0.003s
> - without DMA (-global fw_cfg.dma_enabled=off): sys 0m7.674s
>
> FW_CFG_FILE_DIR (0x19) is the only "file" that is read during kernel
> boot to populate sysfs qemu_fw_cfg directory, and it is quite
> small (1-2kb). Since it does not expose itself, in order to measure
> the time it takes to read such small file, I took a comparable sized
> file of 2048 bytes and exposed it (-fw_cfg test,file=file with a
> modified read_raw enabling DMA)
>
> # perf stat -r 100 cat /sys/firmware/qemu_fw_cfg/by_name/test/raw >/dev/null
> - with DMA:
> 0.636037 task-clock (msec) # 0.141 CPUs utilized ( +- 1.19% )
> - without DMA:
> 6.430128 task-clock (msec) # 0.622 CPUs utilized ( +- 0.22% )
>
> That's a few msec saved during boot by enabling DMA read (the gain
> would be more substantial if other & bigger fw-cfg entries are read by
> others from sysfs, unfortunately, it's not clear if we can always
> enable DMA there)
>
> Signed-off-by: Marc-André Lureau <marcandre.lureau@...hat.com>
> ---
> drivers/firmware/qemu_fw_cfg.c | 61 ++++++++++++++++++++++++++++++++++--------
> 1 file changed, 50 insertions(+), 11 deletions(-)
>
> diff --git a/drivers/firmware/qemu_fw_cfg.c b/drivers/firmware/qemu_fw_cfg.c
> index 3015e77aebca..94df57e9be66 100644
> --- a/drivers/firmware/qemu_fw_cfg.c
> +++ b/drivers/firmware/qemu_fw_cfg.c
> @@ -124,12 +124,47 @@ static ssize_t fw_cfg_dma_transfer(void *address, u32 length, u32 control)
> return ret;
> }
>
> +/* with acpi & dev locks taken */
> +static ssize_t fw_cfg_read_blob_dma(u16 key,
> + void *buf, loff_t pos, size_t count)
> +{
> + ssize_t ret;
> +
> + if (pos == 0) {
> + ret = fw_cfg_dma_transfer(buf, count, key << 16
> + | FW_CFG_DMA_CTL_SELECT
> + | FW_CFG_DMA_CTL_READ);
> + } else {
> + fw_cfg_sel_endianness(key);
> + ret = fw_cfg_dma_transfer(NULL, pos, FW_CFG_DMA_CTL_SKIP);
> + if (ret < 0)
> + return ret;
> + ret = fw_cfg_dma_transfer(buf, count,
> + FW_CFG_DMA_CTL_READ);
> + }
> +
> + return ret;
> +}
> +
> +/* with acpi & dev locks taken */
> +static ssize_t fw_cfg_read_blob_io(u16 key,
> + void *buf, loff_t pos, size_t count)
> +{
> + fw_cfg_sel_endianness(key);
> + while (pos-- > 0)
> + ioread8(fw_cfg_reg_data);
> + ioread8_rep(fw_cfg_reg_data, buf, count);
> + return count;
> +}
> +
> /* read chunk of given fw_cfg blob (caller responsible for sanity-check) */
> static ssize_t fw_cfg_read_blob(u16 key,
> - void *buf, loff_t pos, size_t count)
> + void *buf, loff_t pos, size_t count,
> + bool dma)
> {
> u32 glk = -1U;
> acpi_status status;
> + ssize_t ret;
>
> /* If we have ACPI, ensure mutual exclusion against any potential
> * device access by the firmware, e.g. via AML methods:
so this adds a dma flag to fw_cfg_read_blob.
> @@ -143,14 +178,17 @@ static ssize_t fw_cfg_read_blob(u16 key,
> }
>
> mutex_lock(&fw_cfg_dev_lock);
> - fw_cfg_sel_endianness(key);
> - while (pos-- > 0)
> - ioread8(fw_cfg_reg_data);
> - ioread8_rep(fw_cfg_reg_data, buf, count);
> + if (dma && fw_cfg_dma_enabled()) {
> + ret = fw_cfg_read_blob_dma(key, buf, pos, count);
> + } else {
> + ret = fw_cfg_read_blob_io(key, buf, pos, count);
> + }
> +
> mutex_unlock(&fw_cfg_dev_lock);
>
> acpi_release_global_lock(glk);
> - return count;
> +
> + return ret;
> }
>
> #ifdef CONFIG_CRASH_CORE
If set to false it does io, if set to true it does dma.
I would prefer passing an accessor function pointer
since that's clearer than true/false.
> @@ -284,7 +322,7 @@ static int fw_cfg_do_platform_probe(struct platform_device *pdev)
>
> /* verify fw_cfg device signature */
> if (fw_cfg_read_blob(FW_CFG_SIGNATURE, sig,
> - 0, FW_CFG_SIG_SIZE) < 0 ||
> + 0, FW_CFG_SIG_SIZE, false) < 0 ||
> memcmp(sig, "QEMU", FW_CFG_SIG_SIZE) != 0) {
> fw_cfg_io_cleanup();
> return -ENODEV;
> @@ -468,7 +506,8 @@ static ssize_t fw_cfg_sysfs_read_raw(struct file *filp, struct kobject *kobj,
> if (count > entry->size - pos)
> count = entry->size - pos;
>
> - return fw_cfg_read_blob(entry->select, buf, pos, count);
> + /* do not use DMA, virt_to_phys(buf) might not be ok */
> + return fw_cfg_read_blob(entry->select, buf, pos, count, false);
> }
>
> static struct bin_attribute fw_cfg_sysfs_attr_raw = {
> @@ -634,7 +673,7 @@ static int fw_cfg_register_dir_entries(void)
> size_t dir_size;
>
> ret = fw_cfg_read_blob(FW_CFG_FILE_DIR, &files_count,
> - 0, sizeof(files_count));
> + 0, sizeof(files_count), false);
> if (ret < 0)
> return ret;
>
> @@ -646,7 +685,7 @@ static int fw_cfg_register_dir_entries(void)
> return -ENOMEM;
>
> ret = fw_cfg_read_blob(FW_CFG_FILE_DIR, dir,
> - sizeof(files_count), dir_size);
> + sizeof(files_count), dir_size, false);
> if (ret < 0)
> goto end;
>
> @@ -697,7 +736,7 @@ static int fw_cfg_sysfs_probe(struct platform_device *pdev)
> goto err_probe;
>
> /* get revision number, add matching top-level attribute */
> - err = fw_cfg_read_blob(FW_CFG_ID, &rev, 0, sizeof(rev));
> + err = fw_cfg_read_blob(FW_CFG_ID, &rev, 0, sizeof(rev), false);
> if (err < 0)
> goto err_probe;
Looks like all callers pass in false as parameter.
Given this, how can this speed up any operations?
Are you sure you tested this properly?
> --
> 2.16.1.73.g5832b7e9f2
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