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SubjectRe: [PATCH v13 4/4] RFC: fw_cfg: do DMA read operation
Hi

On Mon, Feb 12, 2018 at 4:30 AM, Michael S. Tsirkin <mst@redhat.com> wrote:
> On Wed, Feb 07, 2018 at 02:35:25AM +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@redhat.com>
>> ---
>> drivers/firmware/qemu_fw_cfg.c | 47 ++++++++++++++++++++++++++++++------------
>> 1 file changed, 34 insertions(+), 13 deletions(-)
>>
>> diff --git a/drivers/firmware/qemu_fw_cfg.c b/drivers/firmware/qemu_fw_cfg.c
>> index fd576ba7b337..3721dc868a2b 100644
>> --- a/drivers/firmware/qemu_fw_cfg.c
>> +++ b/drivers/firmware/qemu_fw_cfg.c
>> @@ -150,11 +150,13 @@ static ssize_t fw_cfg_dma_transfer(void *address, u32 length, u32 control)
>> }
>>
>> /* 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:
>> @@ -164,17 +166,36 @@ 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()) {
>> + 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)
>> + goto end;
>> + ret = fw_cfg_dma_transfer(buf, count,
>> + FW_CFG_DMA_CTL_READ);
>> + }
>> + } else {
>> + 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);
>> + }
>> +
>> +end:
>> mutex_unlock(&fw_cfg_dev_lock);
>>
>> acpi_release_global_lock(glk);
>> +
>> + return ret;
>> }
>>
>
> These two functions share no common code at all.
> Pls name the dma one fw_cfg_dma_read or something like this,
> cleaner than a flag.

They share arguments, ACPI locking, error handling, cleanup. But they
also allow to abstract read over dma-capable and non-dma capable. I'll
split both cases in two functions.

>
>> #ifdef CONFIG_CRASH_CORE
>> @@ -307,7 +328,7 @@ 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);
>> + fw_cfg_read_blob(FW_CFG_SIGNATURE, sig, 0, FW_CFG_SIG_SIZE, false);
>> if (memcmp(sig, "QEMU", FW_CFG_SIG_SIZE) != 0) {
>> fw_cfg_io_cleanup();
>> return -ENODEV;
>
> BTW it looks like fw_cfg_read_blob can fail and that failure isn't
> handled properly here.

ok

>
>> @@ -494,8 +515,8 @@ static ssize_t fw_cfg_sysfs_read_raw(struct file *filp, struct kobject *kobj,
>> if (count > entry->f.size - pos)
>> count = entry->f.size - pos;
>>
>> - fw_cfg_read_blob(entry->f.select, buf, pos, count);
>> - return count;
>> + /* do not use DMA, virt_to_phys(buf) might not be ok */
>> + return fw_cfg_read_blob(entry->f.select, buf, pos, count, false);
>> }
>>
>> static struct bin_attribute fw_cfg_sysfs_attr_raw = {
>> @@ -656,7 +677,7 @@ 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, &count, 0, sizeof(count));
>> + fw_cfg_read_blob(FW_CFG_FILE_DIR, &count, 0, sizeof(count), false);
>> count = be32_to_cpu(count);
>> dir_size = count * sizeof(struct fw_cfg_file);
>>
>> @@ -664,7 +685,7 @@ static int fw_cfg_register_dir_entries(void)
>> if (!dir)
>> return -ENOMEM;
>>
>> - fw_cfg_read_blob(FW_CFG_FILE_DIR, dir, sizeof(count), dir_size);
>> + fw_cfg_read_blob(FW_CFG_FILE_DIR, dir, sizeof(count), dir_size, true);
>>
>> for (i = 0; i < count; i++) {
>> dir[i].size = be32_to_cpu(dir[i].size);
>> @@ -713,7 +734,7 @@ 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, &fw_cfg_rev, 0, sizeof(fw_cfg_rev));
>> + fw_cfg_read_blob(FW_CFG_ID, &fw_cfg_rev, 0, sizeof(fw_cfg_rev), false);
>> fw_cfg_rev = le32_to_cpu(fw_cfg_rev);
>> err = sysfs_create_file(fw_cfg_top_ko, &fw_cfg_rev_attr.attr);
>> if (err)
>> --
>> 2.16.1.73.g5832b7e9f2

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