Re: [PATCH v15 11/11] RFC: fw_cfg: do DMA read operation

["Marc-André Lureau" <marcandre.lureau@redhat.com>]

Hi

On Tue, Feb 27, 2018 at 1:04 AM, Michael S. Tsirkin <mst@redhat.com> wrote:
> 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@redhat.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.

ok

>
>> @@ -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?


I did modify read_raw to conduct testing ( the part "with a
modified read_raw enabling DMA" should be before, updating commit message).

>
>> --
>> 2.16.1.73.g5832b7e9f2