[Bug 1868527] [NEW] alignment may overlap the TLB flags

[Bug 1868527] [NEW] alignment may overlap the TLB flags

[Hansni Bu]

Public bug reported:

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.

** Affects: qemu
     Importance: Undecided
         Status: New

-- 
You received this bug notification because you are a member of qemu-
devel-ml, which is subscribed to QEMU.
https://bugs.launchpad.net/bugs/1868527

Title:
  alignment may overlap the TLB flags

Status in QEMU:
  New

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.

To manage notifications about this bug go to:
https://bugs.launchpad.net/qemu/+bug/1868527/+subscriptions

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

[Richard Henderson]

That is of course completely dependent on the target page size.  So,
yes, a target with a very small page size cannot use large alignments.
The assert makes sure.

Is this comment simply by inspection, or did you have an actual bug to
report?

** Changed in: qemu
       Status: New => Incomplete

-- 
You received this bug notification because you are a member of qemu-
devel-ml, which is subscribed to QEMU.
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.

To manage notifications about this bug go to:
https://bugs.launchpad.net/qemu/+bug/1868527/+subscriptions

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

[Hansni Bu]

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

-- 
You received this bug notification because you are a member of qemu-
devel-ml, which is subscribed to QEMU.
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.

To manage notifications about this bug go to:
https://bugs.launchpad.net/qemu/+bug/1868527/+subscriptions

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

[Hansni Bu]

** Changed in: qemu
       Status: Incomplete => Invalid

-- 
You received this bug notification because you are a member of qemu-
devel-ml, which is subscribed to QEMU.
https://bugs.launchpad.net/bugs/1868527

Title:
  alignment may overlap the TLB flags

Status in QEMU:
  Invalid

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.

To manage notifications about this bug go to:
https://bugs.launchpad.net/qemu/+bug/1868527/+subscriptions