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

["Michael S. Tsirkin" <mst@redhat.com>]

On Wed, Feb 28, 2018 at 01:27:02PM +0100, Marc-André Lureau wrote:
> 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).

So this patch does nothing, it's just infrastructure so DMA can be
enabled in the future - is that right?

> >
> >> --
> >> 2.16.1.73.g5832b7e9f2