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List:       qemu-devel
Subject:    Re: [PATCH 2/3] target/hppa: mask offset bits in gva
From:       Sven Schnelle <svens () stackframe ! org>
Date:       2024-03-28 21:03:27
Message-ID: 87jzlm1334.fsf () t14 ! stackframe ! org
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Richard Henderson <richard.henderson@linaro.org> writes:

> On 3/23/24 22:09, Sven Schnelle wrote:
>> The CPU seems to mask a few bits in the offset when running
>> under HP-UX. ISR/IOR register contents for an address in
>> the processor HPA (0xfffffffffffa0000) on my C8000 and J6750:
>> running on Linux: 000000003fffffff c0000000fffa0500
>> running on HP-UX: 00000000301fffff c0000000fffa0500
>> I haven't found how this is switched (guess some diag in the
>> firmware), but linux + seabios seems to handle that as well,
>> so lets mask out the additional bits.
>> Signed-off-by: Sven Schnelle <svens@stackframe.org>
>> [..]
> [..]
> Though my argument would suggest the mask should be 0xff for the
> 40-bit physical address, which is not what you see at all, so perhaps
> the thing is moot.  I am at a loss to explain why or how HP-UX gets a
> 7-bit hole in the ISR result.
>
> On the other hand, there are some not-well-documented shenanigans (aka
> implementation defined behaviour) between Figure H-8 and Figure H-11,
> where the 62-bit absolute address is expanded to a 64-bit logical
> physical address and then compacted to a 40-bit implementation
> physical address.
>
> We've already got hacks in place for this in hppa_abs_to_phys_pa2_w1,
> which just truncates everything down to 40 bits.  But that's probably
> not what the processor is really doing.

I looked into this again, and it's caused by Space-ID hashing. HP-UX asks
PDC/Firmware how many bits are used for the hashing. seabios returns
zero, in which case HP-UX uses a default mask of 0xf01fffffffffffff.
By modifying seabios, i can make HP-UX use the appropriate mask, but
switching of SpaceID hashing entirely is impossible. The reason why
the CPU doesn't strip the bits when running linux is that Linux switches
of Space-ID hashing early in the startup code (before mm gets
initialized).

My J6750 Firmware only returns two values: 0 when Space-ID hashing is
off, 0xfe0 when it is enabled. This is hardcoded in the firmware - the
only thing PDC checks is a bit in Debug Register 2, which enables
Space-ID hashing. 0xfe0 matches the 0xf01f... mask used by HP-UX
pretty well.

So if qemu wants to run 64 Bit HP-UX the proper way, i guess it needs
to implement Space-ID hashing.

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