On 11/04/15 17:35, Gabriel L. Somlo wrote:
> On Wed, Nov 04, 2015 at 04:04:09PM +0100, Laszlo Ersek wrote:
>> On 11/03/15 22:40, Gabriel L. Somlo wrote:
>>> Introduce fw_cfg_data_read(), a generic read method which works
>>> on all access widths (1 through 8 bytes, inclusive), and can be
>>> used during both IOPort and MMIO read accesses.
>>>
>>> To maintain legibility, only fw_cfg_data_mem_read() (the MMIO
>>> data read method) is replaced by this patch. The new method
>>> essentially unwinds the fw_cfg_data_mem_read() + fw_cfg_read()
>>> combo, but without unnecessarily repeating all the validity
>>> checks performed by the latter on each byte being read.
>>>
>>> This patch also modifies the trace_fw_cfg_read prototype to
>>> accept a 64-bit value argument, allowing it to work properly
>>> with the new read method, but also remain backward compatible
>>> with existing call sites.
>>>
>>> Cc: Laszlo Ersek <ler...@redhat.com>
>>> Cc: Gerd Hoffmann <kra...@redhat.com>
>>> Cc: Marc Marí <mar...@redhat.com>
>>> Signed-off-by: Gabriel Somlo <so...@cmu.edu>
>>> ---
>>> hw/nvram/fw_cfg.c | 44 ++++++++++++++++++++++++++++++--------------
>>> trace-events | 2 +-
>>> 2 files changed, 31 insertions(+), 15 deletions(-)
>>>
>>> diff --git a/hw/nvram/fw_cfg.c b/hw/nvram/fw_cfg.c
>>> index 046fa74..9e01b46 100644
>>> --- a/hw/nvram/fw_cfg.c
>>> +++ b/hw/nvram/fw_cfg.c
>>> @@ -274,6 +274,35 @@ static int fw_cfg_select(FWCfgState *s, uint16_t key)
>>> return ret;
>>> }
>>>
>>> +static uint64_t fw_cfg_data_read(void *opaque, hwaddr addr, unsigned size)
>>> +{
>>> + FWCfgState *s = opaque;
>>> + int arch = !!(s->cur_entry & FW_CFG_ARCH_LOCAL);
>>> + FWCfgEntry *e = (s->cur_entry == FW_CFG_INVALID) ? NULL :
>>> + &s->entries[arch][s->cur_entry & FW_CFG_ENTRY_MASK];
>>> + uint64_t value = 0;
>>> +
>>> + assert(size <= sizeof(value));
>>> + if (s->cur_entry != FW_CFG_INVALID && e->data) {
>>> + /* The least significant 'size' bytes of the return value are
>>> + * expected to contain a string preserving portion of the item
>>> + * data, padded with zeros to the right in case we run out early.
>>> + */
>>> + while (size && s->cur_offset < e->len) {
>>> + value = (value << 8) | e->data[s->cur_offset++];
>>> + size--;
>>> + }
>>> + /* If size is still not zero, we *did* run out early, so continue
>>> + * left-shifting, to add the appropriate number of padding zeros
>>> + * on the right.
>>> + */
>>> + value <<= 8 * size;
>>> + }
>>> +
>>> + trace_fw_cfg_read(s, value);
>>> + return value;
>>> +}
>>
>> With the wording you proposed in
>> <http://thread.gmane.org/gmane.comp.emulators.qemu/373165/focus=373507>,
>> this looks okay.
>>
>> ... Except my (2a) proposal wasn't entirely correct, and now you get to
>> fix it up for v5. :( Apologies. (It is a different experience when you
>> see the code in full.)
>>
>> Namely, consider the case when this code is entered with:
>>
>> (size == 8 && s->cur_offset == e->len)
>>
>> (Which is possible if the guest makes a qword read access just after
>> reading the full blob.)
>>
>> In this case, the loop won't be entered at all (which is okay), but then
>> you'll have:
>>
>> uint64_t << 64
>>
>> which is undefined behavior. ("If the value of the right operand is
>> negative or is greater than or equal to the width of the promoted left
>> operand, the behavior is undefined.")
>
> Yeah, we're hitting all the corner cases of the C standard, aren't we :)
>
>> So please protect the final shift with "if (size < 8)".
>>
>> *Alternatively*, you could restrict the *outer* condition, i.e.,
>>
>> s->cur_entry != FW_CFG_INVALID && e->data
>>
>> with (s->cur_offset < e->len).
>>
>> ... And then you can even replace the "while" with a "do" loop. (Because
>> both (size > 0) and (s->cur_offset < e->len) will be true if the loop is
>> reached at all.)
>>
>> Just the code, without comments:
>>
>> assert(size <= sizeof(value));
>> assert(size > 0);
>> if (s->cur_entry != FW_CFG_INVALID && e->data &&
>> s->cur_offset < e->len) {
>> /* ... */
>> do {
>> value = (value << 8) | e->data[s->cur_offset++];
>> size--;
>> } while (size && s->cur_offset < e->len);
>> /* ... */
>> value <<= 8 * size;
>> }
>>
>> This makes it clear that "size" is strictly smaller than sizeof(value)
>> when the shift is reached.
>>
>> I'll let you choose between the two alternatives. :)
>
> I like the do/while idea, so here's the new function:
>
> +static uint64_t fw_cfg_data_read(void *opaque, hwaddr addr, unsigned size)
> +{
> + FWCfgState *s = opaque;
> + int arch = !!(s->cur_entry & FW_CFG_ARCH_LOCAL);
> + FWCfgEntry *e = (s->cur_entry == FW_CFG_INVALID) ? NULL :
> + &s->entries[arch][s->cur_entry & FW_CFG_ENTRY_MASK];
> + uint64_t value = 0;
> +
> + assert(size > 0 && size <= sizeof(value));
It's a matter of taste, and I won't insist at all, just mention that I
didn't write those two assert()s as separate statements :)
Namely, with a conjunction (P1 && P2 && ... && Pn), you have the
possibility to spell the assertion as:
assert(P1);
assert(P2);
...
assert(Pn);
And, if any one of those fails, you will know *which one*. Because the
line number in the "assertion failed" message will tell you.
> + if (s->cur_entry != FW_CFG_INVALID && e->data && s->cur_offset < e->len)
> {
> + /* The least significant 'size' bytes of the return value are
> + * expected to contain a string preserving portion of the item
> + * data, padded with zeros on the right in case we run out early.
> + * In technical terms, we're composing the host-endian representation
> + * of the big endian interpretation of the fw_cfg string.
> + */
> + do {
> + value = (value << 8) | e->data[s->cur_offset++];
> + } while (--size && s->cur_offset < e->len);
> + /* If size is still not zero, we *did* run out early, so continue
> + * left-shifting, to add the appropriate number of padding zeros
> + * on the right.
> + */
> + value <<= 8 * size;
> + }
> +
> + trace_fw_cfg_read(s, value);
> + return value;
> +}
Looks good!
>
>>
>> Thanks, and I'm sorry.
>
> Thank you, and no worries -- after all, what's *my* excuse for not
> catching it ? :)
"No interest in language lawyering", perhaps? :)
> Guess I'll put a low-pass filter on blasting out v5, given how this is
> a "target rich environment" for subtle C standard violations :)
I think I'm ready to give my R-b for the final missing piece. (Not sure
if others would like to comment as well, on v4.) Your call :)
Cheers
Laszlo
>
> Cheers,
> --Gabriel
>
>>
>>> +
>>> static uint8_t fw_cfg_read(FWCfgState *s)
>>> {
>>> int arch = !!(s->cur_entry & FW_CFG_ARCH_LOCAL);
>>> @@ -291,19 +320,6 @@ static uint8_t fw_cfg_read(FWCfgState *s)
>>> return ret;
>>> }
>>>
>>> -static uint64_t fw_cfg_data_mem_read(void *opaque, hwaddr addr,
>>> - unsigned size)
>>> -{
>>> - FWCfgState *s = opaque;
>>> - uint64_t value = 0;
>>> - unsigned i;
>>> -
>>> - for (i = 0; i < size; ++i) {
>>> - value = (value << 8) | fw_cfg_read(s);
>>> - }
>>> - return value;
>>> -}
>>> -
>>> static void fw_cfg_data_mem_write(void *opaque, hwaddr addr,
>>> uint64_t value, unsigned size)
>>> {
>>> @@ -485,7 +501,7 @@ static const MemoryRegionOps fw_cfg_ctl_mem_ops = {
>>> };
>>>
>>> static const MemoryRegionOps fw_cfg_data_mem_ops = {
>>> - .read = fw_cfg_data_mem_read,
>>> + .read = fw_cfg_data_read,
>>> .write = fw_cfg_data_mem_write,
>>> .endianness = DEVICE_BIG_ENDIAN,
>>> .valid = {
>>> diff --git a/trace-events b/trace-events
>>> index 72136b9..5073040 100644
>>> --- a/trace-events
>>> +++ b/trace-events
>>> @@ -196,7 +196,7 @@ ecc_diag_mem_readb(uint64_t addr, uint32_t ret) "Read
>>> diagnostic %"PRId64"= %02x
>>>
>>> # hw/nvram/fw_cfg.c
>>> fw_cfg_select(void *s, uint16_t key, int ret) "%p key %d = %d"
>>> -fw_cfg_read(void *s, uint8_t ret) "%p = %d"
>>> +fw_cfg_read(void *s, uint64_t ret) "%p = %"PRIx64
>>> fw_cfg_add_file(void *s, int index, char *name, size_t len) "%p #%d: %s
>>> (%zd bytes)"
>>>
>>> # hw/block/hd-geometry.c
>>>
>>
