Re: [SeaBIOS] [PATCH v7 7/8] bootdevice: FW_CFG interface for LCHS values

[John Snow]

On 9/26/19 5:57 AM, Philippe Mathieu-Daudé wrote:
> Hi Sam,
> 
> On 9/25/19 1:06 PM, Sam Eiderman wrote:
>> From: Sam Eiderman <address@hidden>
>>
>> Using fw_cfg, supply logical CHS values directly from QEMU to the BIOS.
>>
>> Non-standard logical geometries break under QEMU.
>>
>> A virtual disk which contains an operating system which depends on
>> logical geometries (consistent values being reported from BIOS INT13
>> AH=08) will most likely break under QEMU/SeaBIOS if it has non-standard
>> logical geometries - for example 56 SPT (sectors per track).
>> No matter what QEMU will report - SeaBIOS, for large enough disks - will
>> use LBA translation, which will report 63 SPT instead.
>>
>> In addition we cannot force SeaBIOS to rely on physical geometries at
>> all. A virtio-blk-pci virtual disk with 255 phyiscal heads cannot
>> report more than 16 physical heads when moved to an IDE controller,
>> since the ATA spec allows a maximum of 16 heads - this is an artifact of
>> virtualization.
>>
>> By supplying the logical geometries directly we are able to support such
>> "exotic" disks.
>>
>> We serialize this information in a similar way to the "bootorder"
>> interface.
>> The new fw_cfg entry is "bios-geometry".
>>
>> Reviewed-by: Karl Heubaum <address@hidden>
>> Reviewed-by: Arbel Moshe <address@hidden>
>> Signed-off-by: Sam Eiderman <address@hidden>
>> ---
>>  bootdevice.c            | 32 ++++++++++++++++++++++++++++++++
>>  hw/nvram/fw_cfg.c       | 14 +++++++++++---
>>  include/sysemu/sysemu.h |  1 +
>>  3 files changed, 44 insertions(+), 3 deletions(-)
>>
>> diff --git a/bootdevice.c b/bootdevice.c
>> index 2b12fb85a4..b034ad7bdc 100644
>> --- a/bootdevice.c
>> +++ b/bootdevice.c
>> @@ -405,3 +405,35 @@ void del_boot_device_lchs(DeviceState *dev, const char 
>> *suffix)
>>          }
>>      }
>>  }
>> +
>> +/* Serialized as: (device name\0 + lchs struct) x devices */
>> +char *get_boot_devices_lchs_list(size_t *size)
>> +{
>> +    FWLCHSEntry *i;
>> +    size_t total = 0;
>> +    char *list = NULL;
>> +
>> +    QTAILQ_FOREACH(i, &fw_lchs, link) {
>> +        char *bootpath;
>> +        char *chs_string;
>> +        size_t len;
>> +
>> +        bootpath = get_boot_device_path(i->dev, false, i->suffix);
>> +        chs_string = g_strdup_printf("%s %" PRIu32 " %" PRIu32 " %" PRIu32,
>> +                                     bootpath, i->lcyls, i->lheads, 
>> i->lsecs);
> 
> Hmm maybe we can g_free(bootpath) directly here.
> 

I think it's okay to do it at the bottom of the loop. No real benefit to
being that eager to free resources in my mind. I expect setup at the top
of a block and teardown at the bottom of a block.

Trying to do too much in the middle gets messy in my opinion, not that
it seems to matter here.

>> +
>> +        if (total) {
>> +            list[total - 1] = '\n';
>> +        }
>> +        len = strlen(chs_string) + 1;
>> +        list = g_realloc(list, total + len);
>> +        memcpy(&list[total], chs_string, len);
>> +        total += len;
>> +        g_free(chs_string);
>> +        g_free(bootpath);
>> +    }
>> +
>> +    *size = total;
>> +
>> +    return list;
>> +}
>> diff --git a/hw/nvram/fw_cfg.c b/hw/nvram/fw_cfg.c
>> index 7dc3ac378e..18aff658c0 100644
>> --- a/hw/nvram/fw_cfg.c
>> +++ b/hw/nvram/fw_cfg.c
>> @@ -920,13 +920,21 @@ void *fw_cfg_modify_file(FWCfgState *s, const char 
>> *filename,
>>  
>>  static void fw_cfg_machine_reset(void *opaque)
>>  {
>> +    MachineClass *mc = MACHINE_GET_CLASS(qdev_get_machine());
>> +    FWCfgState *s = opaque;
>>      void *ptr;
>>      size_t len;
>> -    FWCfgState *s = opaque;
>> -    char *bootindex = get_boot_devices_list(&len);
>> +    char *buf;
>>  
>> -    ptr = fw_cfg_modify_file(s, "bootorder", (uint8_t *)bootindex, len);
>> +    buf = get_boot_devices_list(&len);
>> +    ptr = fw_cfg_modify_file(s, "bootorder", (uint8_t *)buf, len);
>>      g_free(ptr);
>> +
>> +    if (!mc->legacy_fw_cfg_order) {
>> +        buf = get_boot_devices_lchs_list(&len);
>> +        ptr = fw_cfg_modify_file(s, "bios-geometry", (uint8_t *)buf, len);
> 
> OK. Can you add a test in tests/fw_cfg-test.c please?
> 

:D

>> +        g_free(ptr);
>> +    }
>>  }
>>  
>>  static void fw_cfg_machine_ready(struct Notifier *n, void *data)
>> diff --git a/include/sysemu/sysemu.h b/include/sysemu/sysemu.h
>> index 5bc5c79cbc..80c57fdc4e 100644
>> --- a/include/sysemu/sysemu.h
>> +++ b/include/sysemu/sysemu.h
>> @@ -106,6 +106,7 @@ void validate_bootdevices(const char *devices, Error 
>> **errp);
>>  void add_boot_device_lchs(DeviceState *dev, const char *suffix,
>>                            uint32_t lcyls, uint32_t lheads, uint32_t lsecs);
>>  void del_boot_device_lchs(DeviceState *dev, const char *suffix);
>> +char *get_boot_devices_lchs_list(size_t *size);
> 
> Please add some documentation. At least 'size' must be non-NULL.
> 

Sure; but I wasn't going to gate on it because this series went unloved
for so long. At this point, a follow-up patch is fine.

> Ideally you should add doc for the other functions added in 3/8
> "bootdevice: Add interface to gather LCHS" too.
> 

Same thing here.

> John, what do you think about extracting the *boot_device* functions out
> of "sysemu.h"?
> 

Potentially worthwhile; but not critical at the moment. The source tree
is not the best-organized thing as-is and I don't think it's fair to
hold this series up for much longer for nice-to-haves, ultimately.

More targeted improvements might avoid the "whose responsibility is it
to stage this?" hot potato we played with this one; so I'd rather have
smaller follow-up patches handled by the respective maintainers.

> Thanks,
> 
> Phil.
> 
Thanks for the reviews :)

--js