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[Bug 812398] Re: powerpc 7450 MMU initialization broken

From: till
Subject: [Bug 812398] Re: powerpc 7450 MMU initialization broken
Date: Sat, 08 Aug 2020 10:42:58 -0000

>From looking at the source code of 5.1.0-rc3
(target/ppc/translate_init.inc.c) it seems that this is still an issue.

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  powerpc 7450 MMU initialization broken

Status in QEMU:

Bug description:
  The 7540 family of PPCs' MMU can update TLBs using hardware search
  (like a 604 or 7400) but also using a software algorithm. The
  mechanism used is defined by HID0[STEN].

  By default (CPU reset) HID0 is set to 0x80000000 (BTW; another small bug, 
qemu doesn't set the hardwired MSB), hence
  the software-table lookup feature is *disabled*. However, the default (and 
immutable) 'mmu_model' for this CPU family is POWERC_MMU_SOFT_74XX which choses 
the soft TLB replacement scheme.

  To fix this:

  1) the initial mmu_model for the 7450 family (includes 7441, 7445, 7451, 
7455, 7457, 7447, 7448) should be: POWERPC_MMU_32B
  2) when HID0[STEN] is written then the mmu_model should be changed 
accordingly (I'm not familiar enough with the qemu internal state to judge if 
any cached state would have to be updated).

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