On 24/09/15 09:11, Jan Beulich wrote:
On 23.09.15 at 20:34, <andrew.coop...@citrix.com> wrote:
On 22/09/15 14:10, Jan Beulich wrote:
+ for ( offs = 2; offs < 8; offs <<= 1 )
+ {
+ bool_t read = 1;
+
+ for ( i = RTC_REG_D + 1; i < 0x80; ++i )
+ {
+ uint8_t normal, alt;
+ unsigned long flags;
+
+ if ( i == acpi_gbl_FADT.century )
+ continue;
+
+ spin_lock_irqsave(&rtc_lock, flags);
+
+ normal = CMOS_READ(i);
+ if ( inb(RTC_PORT(offs)) != i )
+ read = 0;
+
+ alt = inb(RTC_PORT(offs + 1));
+
+ spin_unlock_irqrestore(&rtc_lock, flags);
+
+ if ( normal != alt )
+ break;
Even with a manual to hand, this logic is quite hard to understand.
Furthermore, I still cant spot how your r/w vs w/o logic is supposed to
work. It doesn't check the writability of the alias, but of the aliases
index.
But that's the only interesting thing. A w/o alias would be rather
strange. It's the index register that traditionally hasn't been
readable, but has got means added in some chipsets to be read
back. For the moment we don't make use of this information
anyway, i.e. it's more a thing usable for validation that what the
logic determines matches with how the chipset is documented to
behave (if someone wanted to check that).
However, it is not robust to the system servicing an SMI and altering
the CMOS ram in the middle of this loop. Such a modification would
cause the loop to believe that this specific 'offs' is not an alias even
when it actually is.
One option would be to reread the non-aliased port again, but that would
add yet more io reads.
SMI is always a problem. Even if we re-read the register, we still
couldn't be sure we haven't got hit by another SMI. Considering
the index/data port access model I would anyway consider it quite
bad for firmware to modify the CMOS in an SMI.
When one core is executing the SMM handler, all other cores are held
paused. (There tends to be some rather complicated cross-socket logic
to make this happen properly).
It is safe for the SMM handler to use CMOS if it returns the index
register back to how it found it. Furthermore, I am willing to bet that
there real SMM handlers out there which do do this.
~Andrew
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