RE: [RFC PATCH v2] ACPICA / Hardware: Fix old register check in acpi_hw_get_access_bit_width()

[Zheng, Lv]

Hi,

> From: Boris Ostrovsky [mailto:boris.ostrovsky@xxxxxxxxxx]
> Subject: Re: [RFC PATCH v2] ACPICA / Hardware: Fix old register check in
> acpi_hw_get_access_bit_width()
> 
> On 05/31/2016 10:36 AM, Mike Marshall wrote:
> > Hi Lv...
> >
> > I was dead in the water before this patch, qemu-kvm would crash
> > right away, now everything seems to work great again, thanks! From
> > my perspective this fixes the c3bc26d problem.
> >
> > Acked-by: Mike Marshall <hubcap@xxxxxxxxxxxx>
> >
> > -Mike
> >
> > On Tue, May 31, 2016 at 3:13 AM, Zheng, Lv <lv.zheng@xxxxxxxxx>
> wrote:
> >> Hi, Boris and Mike
> >>
> >> Please help to validate if this version can also fix your issues.
> >> After enumerating the possible cases, I realized that the address check
> might not be necessary.
> >> But we need a max_bit_width check in this function to make it
> prepared for a future usage in acpi_read()/acpi_write().
> >> Thanks in advance.
> 
> You can add
>   Tested-by: Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>
[Lv Zheng] 
Great!

> 
> although this still allows us to access bytes that we are not supposed to.
> 
> You may be able to calculate access width as something like
> 
>    min (max_bit_width,
>            ACPI_ROUND_UP((ACPI_ROUND_DOWN(reg->bit_offset, 8) +
> reg->bit_width),
>            8);
[Lv Zheng] 
This looks reasonable to me.
And I actually was considering to add such kind of code before.

But since we have been working with ACPICA for so many years without supporting reg->bit_offset.
We probably can make sure that there is no real platform setting reg->bit_offset to the values other than 0.

It is likely that the real platforms that want to utilize "access size" are using it correctly, so we actually needn't do any optimization in acpi_hw_get_access_bit_size() except returning the default one.
The code I put here (which looks like an optimization) is meant to create a workaround for the bug reported against acpi_tb_init_generic_address().

Thus IMO, we could do less aggressive approach and do further changes unless we can see real cases.

Cheers
-Lv

> 
> -boris
> 
> 
> >> Best regards
> >> -Lv
> >>
> >>> From: Zheng, Lv
> >>> Subject: [RFC PATCH v2] ACPICA / Hardware: Fix old register check in
> >>> acpi_hw_get_access_bit_width()
> >>>
> >>> The address check in acpi_hw_get_access_bit_width() should be byte
> >>> width
> >>> based, not bit width based. This patch fixes this mistake.
> >>>
> >>> For those who want to review acpi_hw_access_bit_width(), here is the
> >>> concerns and the design details of the function:
> >>>
> >>> It is supposed that the GAS Address field should be aligned to the byte
> >>> width indicated by the GAS AccessSize field. Similarly, for the old non
> >>> GAS register, it is supposed that its Address should be aligned to its
> >>> Length.
> >>> For the "AccessSize = 0 (meaning ANY)" case, we try to return the
> >>> maximum
> >>> instruction width (64 for MMIO or 32 for PIO) or the user expected
> access
> >>> bit width (64 for acpi_read()/acpi_write() or 32 for acpi_hw_read()/
> >>> acpi_hw_write()) for futher operation and it is supposed that the GAS
> >>> Address field should always be aligned to the maximum expected
> access
> >>> bit
> >>> width (otherwise it can't be ANY).
> >>>
> >>> The problem is in acpi_tb_init_generic_address(), where the non GAS
> >>> register's Length is converted into the GAS BitWidth field, its Address
> is
> >>> converted into the GAS Address field, and the GAS AccessSize field is
> left
> >>> 0 but most of the register actually cannot be accessed using "ANY"
> >>> accesses.
> >>>
> >>> As a conclusion, when AccessSize = 0 (ANY), the Address should either
> be
> >>> aligned to the BitWidth (wrong conversion) or aligned to 32 (PIO) or
> 64
> >>> (MMIO). Since BitWidth for the wrong conversion is 8,16,32, the
> Address
> >>> of the real GAS should always be aligned to 8,16,32, the address
> alignment
> >>> check is not necessary. But we in fact could enhance the check for a
> future
> >>> case where max_bit_width could be 64 for a PIO access issued from
> >>> acpi_read()/acpi_write().
> >>>
> >>> Fixes: b314a172ee96 ("ACPICA: Hardware: Add optimized access bit
> width
> >>> support")
> >>> Cc: Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>
> >>> Cc: Mike Marshall <hubcap@xxxxxxxxxxxx>
> >>> Suggested-by: Jan Beulich <jbeulich@xxxxxxxx>
> >>> Signed-off-by: Lv Zheng <lv.zheng@xxxxxxxxx>
> >>> ---
> >>>  drivers/acpi/acpica/hwregs.c |   16 +++++++---------
> >>>  1 file changed, 7 insertions(+), 9 deletions(-)
> >>>
> >>> diff --git a/drivers/acpi/acpica/hwregs.c
> b/drivers/acpi/acpica/hwregs.c
> >>> index 0f18dbc..0553c0b 100644
> >>> --- a/drivers/acpi/acpica/hwregs.c
> >>> +++ b/drivers/acpi/acpica/hwregs.c
> >>> @@ -86,24 +86,22 @@ acpi_hw_get_access_bit_width(struct
> >>> acpi_generic_address *reg, u8 max_bit_width)
> >>>       u64 address;
> >>>
> >>>       if (!reg->access_width) {
> >>> +             if (reg->space_id == ACPI_ADR_SPACE_SYSTEM_IO) {
> >>> +                     max_bit_width = 32;
> >>> +             }
> >>>               /*
> >>>                * Detect old register descriptors where only the bit_width
> >>> field
> >>>                * makes senses. The target address is copied to handle
> >>> possible
> >>>                * alignment issues.
> >>>                */
> >>>               ACPI_MOVE_64_TO_64(&address, &reg->address);
> >>> -             if (!reg->bit_offset && reg->bit_width &&
> >>> +             if (reg->bit_width < max_bit_width &&
> >>> +                 !reg->bit_offset && reg->bit_width &&
> >>>                   ACPI_IS_POWER_OF_TWO(reg->bit_width) &&
> >>> -                 ACPI_IS_ALIGNED(reg->bit_width, 8) &&
> >>> -                 ACPI_IS_ALIGNED(address, reg->bit_width)) {
> >>> +                 ACPI_IS_ALIGNED(reg->bit_width, 8)) {
> >>>                       return (reg->bit_width);
> >>> -             } else {
> >>> -                     if (reg->space_id == ACPI_ADR_SPACE_SYSTEM_IO)
> >>> {
> >>> -                             return (32);
> >>> -                     } else {
> >>> -                             return (max_bit_width);
> >>> -                     }
> >>>               }
> >>> +             return (max_bit_width);
> >>>       } else {
> >>>               return (1 << (reg->access_width + 2));
> >>>       }
> >>> --
> >>> 1.7.10