Re: [PATCH v14 9/9] RFC: fw_cfg: do DMA read operation

From: Marc-Andre Lureau
Date: Wed Feb 14 2018 - 11:52:15 EST

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In reply to: Michael S. Tsirkin: "Re: [PATCH v14 9/9] RFC: fw_cfg: do DMA read operation"
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Hi

On Wed, Feb 14, 2018 at 5:48 PM, Michael S. Tsirkin <mst@xxxxxxxxxx> wrote:
> On Wed, Feb 14, 2018 at 03:18:50PM +0100, Marc-AndrÃ 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-AndrÃ Lureau <marcandre.lureau@xxxxxxxxxx>
>> ---
>> drivers/firmware/qemu_fw_cfg.c | 80 ++++++++++++++++++++++++++++++++++--------
>> 1 file changed, 66 insertions(+), 14 deletions(-)
>>
>> diff --git a/drivers/firmware/qemu_fw_cfg.c b/drivers/firmware/qemu_fw_cfg.c
>> index 69939e2529f2..ba9b907a4399 100644
>> --- a/drivers/firmware/qemu_fw_cfg.c
>> +++ b/drivers/firmware/qemu_fw_cfg.c
>> @@ -124,12 +124,46 @@ 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 {
>> + iowrite16(fw_cfg_sel_endianness(key), fw_cfg_reg_ctrl);
>> + 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 void fw_cfg_read_blob_io(u16 key,
>> + void *buf, loff_t pos, size_t count)
>> +{
>> + iowrite16(fw_cfg_sel_endianness(key), fw_cfg_reg_ctrl);
>> + while (pos-- > 0)
>> + ioread8(fw_cfg_reg_data);
>> + ioread8_rep(fw_cfg_reg_data, buf, count);
>> +}
>> +
>> /* read chunk of given fw_cfg blob (caller responsible for sanity-check) */
>> -static inline void fw_cfg_read_blob(u16 key,
>> - void *buf, loff_t pos, size_t count)
>> +static ssize_t fw_cfg_read_blob(u16 key,
>> + void *buf, loff_t pos, size_t count,
>> + bool dma)
>> {
>> u32 glk = -1U;
>> acpi_status status;
>> + ssize_t ret = count;
>>
>> /* If we have ACPI, ensure mutual exclusion against any potential
>> * device access by the firmware, e.g. via AML methods:
>> @@ -139,17 +173,21 @@ static inline void fw_cfg_read_blob(u16 key,
>> /* Should never get here */
>> WARN(1, "fw_cfg_read_blob: Failed to lock ACPI!\n");
>> memset(buf, 0, count);
>> - return;
>> + return -EINVAL;
>> }
>>
>> mutex_lock(&fw_cfg_dev_lock);
>> - iowrite16(fw_cfg_sel_endianness(key), fw_cfg_reg_ctrl);
>> - 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 {
>> + fw_cfg_read_blob_io(key, buf, pos, count);
>
> I'd set ret = count here.

Ok, why not.

>
>> + }
>> +
>> mutex_unlock(&fw_cfg_dev_lock);
>>
>> acpi_release_global_lock(glk);
>> +
>> + return ret;
>> }
>>
>> #ifdef CONFIG_CRASH_CORE
>> @@ -282,8 +320,9 @@ static int fw_cfg_do_platform_probe(struct platform_device *pdev)
>> #endif
>>
>> /* verify fw_cfg device signature */
>> - fw_cfg_read_blob(FW_CFG_SIGNATURE, sig, 0, FW_CFG_SIG_SIZE);
>> - if (memcmp(sig, "QEMU", FW_CFG_SIG_SIZE) != 0) {
>> + if (fw_cfg_read_blob(FW_CFG_SIGNATURE, sig,
>> + 0, FW_CFG_SIG_SIZE, false) < 0 ||
>> + memcmp(sig, "QEMU", FW_CFG_SIG_SIZE) != 0) {
>> fw_cfg_io_cleanup();
>> return -ENODEV;
>> }
>
> Rather than add dead code, how about a promise not to
> fail if dma is disabled? Patch will be smaller then.

Even with dma disabled, you could have a locking bug which was
silently ignored before and is now taken into account.

>
>> @@ -466,8 +505,8 @@ static ssize_t fw_cfg_sysfs_read_raw(struct file *filp, struct kobject *kobj,
>> if (count > entry->size - pos)
>> count = entry->size - pos;
>>
>> - fw_cfg_read_blob(entry->select, buf, pos, count);
>> - return 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 = {
>> @@ -632,7 +671,12 @@ static int fw_cfg_register_dir_entries(void)
>> struct fw_cfg_file *dir;
>> size_t dir_size;
>>
>> - fw_cfg_read_blob(FW_CFG_FILE_DIR, &files.count, 0, sizeof(files.count));
>> + ret = fw_cfg_read_blob(FW_CFG_FILE_DIR, &files.count,
>> + 0, sizeof(files.count), false);
>> + if (ret < 0) {
>> + return ret;
>> + }
>> +
>> count = be32_to_cpu(files.count);
>> dir_size = count * sizeof(struct fw_cfg_file);
>>
>> @@ -640,7 +684,11 @@ static int fw_cfg_register_dir_entries(void)
>> if (!dir)
>> return -ENOMEM;
>>
>> - fw_cfg_read_blob(FW_CFG_FILE_DIR, dir, sizeof(files.count), dir_size);
>> + ret = fw_cfg_read_blob(FW_CFG_FILE_DIR, dir,
>> + sizeof(files.count), dir_size, false);
>> + if (ret < 0) {
>> + goto end;
>> + }
>>
>> for (i = 0; i < count; i++) {
>> ret = fw_cfg_register_file(&dir[i]);
>> @@ -648,6 +696,7 @@ static int fw_cfg_register_dir_entries(void)
>> break;
>> }
>>
>> +end:
>> kfree(dir);
>> return ret;
>> }
>> @@ -688,7 +737,10 @@ static int fw_cfg_sysfs_probe(struct platform_device *pdev)
>> goto err_probe;
>>
>> /* get revision number, add matching top-level attribute */
>> - 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;
>> + }
>> fw_cfg_rev = le32_to_cpu(rev);
>> err = sysfs_create_file(fw_cfg_top_ko, &fw_cfg_rev_attr.attr);
>> if (err)
>> --
>> 2.16.1.73.g5832b7e9f2

Next message: Markus Heiser: "Re: [PATCH 8/8] docs: kernel-doc: Don't mangle literal code blocks in comments"
Previous message: James Hogan: "Re: [PATCH v3 4/8] MIPS: mscc: Add initial support for Microsemi MIPS SoCs"
In reply to: Michael S. Tsirkin: "Re: [PATCH v14 9/9] RFC: fw_cfg: do DMA read operation"
Next in thread: Michael S. Tsirkin: "Re: [PATCH v14 9/9] RFC: fw_cfg: do DMA read operation"
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