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[Bug 1868527] Re: alignment may overlap the TLB flags


From: Hansni Bu
Subject: [Bug 1868527] Re: alignment may overlap the TLB flags
Date: Tue, 24 Mar 2020 01:32:16 -0000

This is an inspection yet.
For ARM SMMU simulation, TARGET_PAGE_BITS_MIN is 10. All low bits of the TLB 
virtual address are used up by TLB flags and alignment flags. It's a little 
crowded.
/*
 * ARMv7 and later CPUs have 4K pages minimum, but ARMv5 and v6
 * have to support 1K tiny pages.
 */
# define TARGET_PAGE_BITS_VARY
# define TARGET_PAGE_BITS_MIN  10

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https://bugs.launchpad.net/bugs/1868527

Title:
  alignment may overlap the TLB flags

Status in QEMU:
  Incomplete

Bug description:
  Hi,
  In QEMU-4.2.0, or git-9b26a610936deaf436af9b7e39e4b7f0a35e4409, alignment may 
overlap the TLB flags. 
  For example, the alignment: MO_ALIGN_32,
      MO_ALIGN_32 = 5 << MO_ASHIFT,
  and the TLB flag: TLB_DISCARD_WRITE
  #define TLB_DISCARD_WRITE   (1 << (TARGET_PAGE_BITS_MIN - 6))

  then, in the function "get_alignment_bits", the assert may fail:

  #if defined(CONFIG_SOFTMMU)
      /* The requested alignment cannot overlap the TLB flags.  */
      tcg_debug_assert((TLB_FLAGS_MASK & ((1 << a) - 1)) == 0);
  #endif

  However, the alignment of MO_ALIGN_32 is not used for now, so the
  assert cannot be triggered in current version. Anyway it seems like a
  potential conflict.

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