Re: [PATCH v13 3/4] fw_cfg: write vmcoreinfo details

[Michael S. Tsirkin]

On Mon, Feb 12, 2018 at 11:04:49AM +0100, Marc-Andre Lureau wrote:
> Hi
> 
> On Mon, Feb 12, 2018 at 4:43 AM, Michael S. Tsirkin <mst@xxxxxxxxxx> wrote:
> > On Wed, Feb 07, 2018 at 02:35:24AM +0100, Marc-André Lureau wrote:
> >> If the "etc/vmcoreinfo" fw_cfg file is present and we are not running
> >> the kdump kernel, write the addr/size of the vmcoreinfo ELF note.
> >>
> >> The DMA operation is expected to run synchronously with today qemu,
> >> but the specification states that it may become async, so we run
> >> "control" field check in a loop for eventual changes.
> >>
> >> Signed-off-by: Marc-André Lureau <marcandre.lureau@xxxxxxxxxx>
> >> ---
> >>  drivers/firmware/qemu_fw_cfg.c | 157 ++++++++++++++++++++++++++++++++++++++++-
> >>  1 file changed, 154 insertions(+), 3 deletions(-)
> >>
> >> diff --git a/drivers/firmware/qemu_fw_cfg.c b/drivers/firmware/qemu_fw_cfg.c
> >> index 740df0df2260..fd576ba7b337 100644
> >> --- a/drivers/firmware/qemu_fw_cfg.c
> >> +++ b/drivers/firmware/qemu_fw_cfg.c
> >> @@ -33,6 +33,9 @@
> >>  #include <linux/slab.h>
> >>  #include <linux/io.h>
> >>  #include <linux/ioport.h>
> >> +#include <linux/delay.h>
> >> +#include <linux/crash_dump.h>
> >> +#include <linux/crash_core.h>
> >>
> >>  MODULE_AUTHOR("Gabriel L. Somlo <somlo@xxxxxxx>");
> >>  MODULE_DESCRIPTION("QEMU fw_cfg sysfs support");
> >> @@ -43,12 +46,24 @@ MODULE_LICENSE("GPL");
> >>  #define FW_CFG_ID         0x01
> >>  #define FW_CFG_FILE_DIR   0x19
> >>
> >> +#define FW_CFG_VERSION_DMA     0x02
> >> +#define FW_CFG_DMA_CTL_ERROR   0x01
> >> +#define FW_CFG_DMA_CTL_READ    0x02
> >> +#define FW_CFG_DMA_CTL_SKIP    0x04
> >> +#define FW_CFG_DMA_CTL_SELECT  0x08
> >> +#define FW_CFG_DMA_CTL_WRITE   0x10
> >> +
> >>  /* size in bytes of fw_cfg signature */
> >>  #define FW_CFG_SIG_SIZE 4
> >>
> >>  /* fw_cfg "file name" is up to 56 characters (including terminating nul) */
> >>  #define FW_CFG_MAX_FILE_PATH 56
> >>
> >> +#define VMCOREINFO_FORMAT_ELF 0x1
> >
> > How about exporting interface parts in include/uapi/linux/ ?
> > QEMU can import it from there then.
> > This is what virtio does.
> 
> Good idea, we didn't have it yet. So this is an additional change.
> I'll work on it. Though, if this should delay more this series, I
> think we should drop it.

It's not a new issue so I'm fine doing this as a separate patch on top
if that helps converge.

> >
> >> +
> >> +/* fw_cfg revision attribute, in /sys/firmware/qemu_fw_cfg top-level dir. */
> >> +static u32 fw_cfg_rev;
> >> +
> >>  /* fw_cfg file directory entry type */
> >>  struct fw_cfg_file {
> >>       u32 size;
> >> @@ -57,6 +72,12 @@ struct fw_cfg_file {
> >>       char name[FW_CFG_MAX_FILE_PATH];
> >>  };
> >>
> >> +struct fw_cfg_dma {
> >> +     u32 control;
> >> +     u32 length;
> >> +     u64 address;
> >> +} __packed;
> >> +
> >
> > you can drop __packed here - it's always aligned properly.
> 
> Isn't it preferable to make that explicit? Fwiw, qemu also declares
> the struct packed in its headers.

qemu has a weird coding style with all kind of theoretical ideas.
This attribute disables structure alignment
rules often making gcc generate more code, which we do not
want in the kernel.

> >
> >>  /* fw_cfg device i/o register addresses */
> >>  static bool fw_cfg_is_mmio;
> >>  static phys_addr_t fw_cfg_p_base;
> >> @@ -75,6 +96,59 @@ static inline u16 fw_cfg_sel_endianness(u16 key)
> >>       return fw_cfg_is_mmio ? cpu_to_be16(key) : cpu_to_le16(key);
> >>  }
> >>
> >> +static inline bool fw_cfg_dma_enabled(void)
> >> +{
> >> +     return fw_cfg_rev & FW_CFG_VERSION_DMA && fw_cfg_reg_dma;
> >
> > Why do you use () with == below but not with && here?
> >
> 
> Let's add them.
> 
> >> +}
> >> +
> >> +/* qemu fw_cfg device is sync today, but spec says it may become async */
> >> +static void fw_cfg_wait_for_control(struct fw_cfg_dma *d)
> >> +{
> >> +     do {
> >> +             u32 ctrl = be32_to_cpu(READ_ONCE(d->control));
> >> +
> >> +             if ((ctrl & ~FW_CFG_DMA_CTL_ERROR) == 0)
> >> +                     return;
> >> +
> >> +             usleep_range(50, 100);
> >> +     } while (true);
> >
> > And you need an smp rmb here.

I'd just do rmb() in fact.

> Could you explain? thanks

See Documentation/memory-barriers.txt
You know that control is valid, but following read of
the structure could be reordered. So you need that barrier there.
Same for write: wmb.


> >
> >> +}
> >> +
> >> +static ssize_t fw_cfg_dma_transfer(void *address, u32 length, u32 control)
> >> +{
> >> +     phys_addr_t dma;
> >> +     struct fw_cfg_dma *d = NULL;
> >> +     ssize_t ret = length;
> >> +
> >> +     d = kmalloc(sizeof(*d), GFP_KERNEL);
> >> +     if (!d) {
> >> +             ret = -ENOMEM;
> >> +             goto end;
> >> +     }
> >> +
> >> +     *d = (struct fw_cfg_dma) {
> >> +             .address = address ? cpu_to_be64(virt_to_phys(address)) : 0,
> >> +             .length = cpu_to_be32(length),
> >> +             .control = cpu_to_be32(control)
> >> +     };
> >> +
> >> +     dma = virt_to_phys(d);
> >
> > Pls add docs on why this DMA bypasses the DMA API.
> 
> Peter said in his patch: "fw_cfg device does not need IOMMU
> protection, so use physical addresses
> always.  That's how QEMU implements fw_cfg.  Otherwise we'll see call
> traces during boot."
> 
> Is that enough justification?

what are the reasons for the traces exactly though?
some kind of explanation should go into comments, and
I think it should be a bit more detailed than just "it doesn't
work otherwise".


> >
> >> +
> >> +     iowrite32be((u64)dma >> 32, fw_cfg_reg_dma);
> >> +     iowrite32be(dma, fw_cfg_reg_dma + 4);
> >> +
> >> +     fw_cfg_wait_for_control(d);
> >> +
> >> +     if (be32_to_cpu(READ_ONCE(d->control)) & FW_CFG_DMA_CTL_ERROR) {
> >> +             ret = -EIO;
> >> +     }
> >> +
> >> +end:
> >> +     kfree(d);
> >> +
> >> +     return ret;
> >> +}
> >> +
> >>  /* 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)
> >> @@ -103,6 +177,47 @@ static inline void fw_cfg_read_blob(u16 key,
> >>       acpi_release_global_lock(glk);
> >>  }
> >>
> >> +#ifdef CONFIG_CRASH_CORE
> >> +/* write chunk of given fw_cfg blob (caller responsible for sanity-check) */
> >> +static ssize_t fw_cfg_write_blob(u16 key,
> >> +                              void *buf, loff_t pos, size_t count)
> >
> > fw_cfg_dma_write seems like a nicer name.
> 
> ok (I used the same naming as fw_cfg_read_blob() for consistency)
> 
> >
> >> +{
> >> +     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:
> >> +      */
> >> +     status = acpi_acquire_global_lock(ACPI_WAIT_FOREVER, &glk);
> >> +     if (ACPI_FAILURE(status) && status != AE_NOT_CONFIGURED) {
> >> +             /* Should never get here */
> >> +             WARN(1, "%s: Failed to lock ACPI!\n", __func__);
> >> +             return -EINVAL;
> >> +     }
> >> +
> >> +     mutex_lock(&fw_cfg_dev_lock);
> >> +     if (pos == 0) {
> >> +             ret = fw_cfg_dma_transfer(buf, count, key << 16
> >> +                                       | FW_CFG_DMA_CTL_SELECT
> >> +                                       | FW_CFG_DMA_CTL_WRITE);
> >> +     } 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_WRITE);
> >> +     }
> >> +
> >> +end:
> >> +     mutex_unlock(&fw_cfg_dev_lock);
> >> +
> >> +     acpi_release_global_lock(glk);
> >> +
> >> +     return ret;
> >> +}
> >> +#endif /* CONFIG_CRASH_CORE */
> >> +
> >>  /* clean up fw_cfg device i/o */
> >>  static void fw_cfg_io_cleanup(void)
> >>  {
> >> @@ -201,9 +316,6 @@ static int fw_cfg_do_platform_probe(struct platform_device *pdev)
> >>       return 0;
> >>  }
> >>
> >> -/* fw_cfg revision attribute, in /sys/firmware/qemu_fw_cfg top-level dir. */
> >> -static u32 fw_cfg_rev;
> >> -
> >>  static ssize_t fw_cfg_showrev(struct kobject *k, struct attribute *a, char *buf)
> >>  {
> >>       return sprintf(buf, "%u\n", fw_cfg_rev);
> >> @@ -224,6 +336,37 @@ struct fw_cfg_sysfs_entry {
> >>       struct list_head list;
> >>  };
> >>
> >> +#ifdef CONFIG_CRASH_CORE
> >> +static ssize_t write_vmcoreinfo(const struct fw_cfg_file *f)
> >
> > why not prefix with fw_cfg here?
> 
> ok
> 
> >
> >> +{
> >> +     struct vmci {
> >> +             __le16 host_format;
> >> +             __le16 guest_format;
> >> +             __le32 size;
> >> +             __le64 paddr;
> >> +     } __packed;
> >
> >
> > No need for the __packed attribute.
> 
> discussed above
> 
> > And pls do not declare structures within functions.
> > Name them sanely and place in a header or near top of file.
> 
> ok
> 
> >
> >> +     static struct vmci *data;
> >> +     ssize_t ret;
> >> +
> >> +     data = kmalloc(sizeof(struct vmci), GFP_KERNEL);
> >> +     if (!data)
> >> +             return -ENOMEM;
> >> +
> >> +     *data = (struct vmci) {
> >> +             .guest_format = cpu_to_le16(VMCOREINFO_FORMAT_ELF),
> >> +             .size = cpu_to_le32(VMCOREINFO_NOTE_SIZE),
> >> +             .paddr = cpu_to_le64(paddr_vmcoreinfo_note())
> >> +     };
> >> +     /* spare ourself reading host format support for now since we
> >> +      * don't know what else to format - host may ignore ours
> >> +      */
> >> +     ret = fw_cfg_write_blob(f->select, data, 0, sizeof(struct vmci));
> >> +
> >> +     kfree(data);
> >> +     return ret;
> >> +}
> >> +#endif /* CONFIG_CRASH_CORE */
> >> +
> >>  /* get fw_cfg_sysfs_entry from kobject member */
> >>  static inline struct fw_cfg_sysfs_entry *to_entry(struct kobject *kobj)
> >>  {
> >> @@ -464,6 +607,14 @@ static int fw_cfg_register_file(const struct fw_cfg_file *f)
> >>       int err;
> >>       struct fw_cfg_sysfs_entry *entry;
> >>
> >> +#ifdef CONFIG_CRASH_CORE
> >> +     if (fw_cfg_dma_enabled() &&
> >> +             strcmp(f->name, "etc/vmcoreinfo") == 0 && !is_kdump_kernel()) {
> >> +             if (write_vmcoreinfo(f) < 0)
> >> +                     pr_warn("fw_cfg: failed to write vmcoreinfo");
> >> +     }
> >> +#endif
> >> +
> >>       /* allocate new entry */
> >>       entry = kzalloc(sizeof(*entry), GFP_KERNEL);
> >>       if (!entry)
> >> --
> >> 2.16.1.73.g5832b7e9f2
> 
> thanks