Re: [Qemu-devel] [PATCH v2] Improve the alignment check infrastructure

[Richard Henderson <rth@twiddle.net>]

On 06/22/2016 05:37 AM, Sergey Sorokin wrote:
> +/* Use this mask to check interception with an alignment mask
> + * in a TCG backend.
> + */
> +#define TLB_FLAGS_MASK  (TLB_INVALID_MASK | TLB_NOTDIRTY | TLB_MMIO)

I think we ought to check this in tcg-op.c, rather than wait until generating
code in the backend.


> --- a/tcg/aarch64/tcg-target.inc.c
> +++ b/tcg/aarch64/tcg-target.inc.c
> @@ -1071,19 +1071,21 @@ static void tcg_out_tlb_read(TCGContext *s, TCGReg addr_reg, TCGMemOp opc,
>      int tlb_offset = is_read ?
>          offsetof(CPUArchState, tlb_table[mem_index][0].addr_read)
>          : offsetof(CPUArchState, tlb_table[mem_index][0].addr_write);
> -    int s_mask = (1 << (opc & MO_SIZE)) - 1;
> +    int a_bits = get_alignment_bits(opc);
>      TCGReg base = TCG_AREG0, x3;
> -    uint64_t tlb_mask;
> +    target_ulong tlb_mask;

Hum.  I had been talking about i386 specifically when changing the type of
tlb_mask.

For aarch64, a quirk in the code generation logic requires that a 32-bit
tlb_mask be sign-extended to 64-bit.  The effect of the actual instruction will
be zero-extension, however.

See is_limm, tcg_out_logicali, and a related comment in tcg_out_movi for
details.  We should probably add a comment here in tlb_read for the next person
that comes along...

> diff --git a/tcg/ppc/tcg-target.inc.c b/tcg/ppc/tcg-target.inc.c
> index da10052..3dc38fa 100644
> --- a/tcg/ppc/tcg-target.inc.c
> +++ b/tcg/ppc/tcg-target.inc.c
> @@ -1399,6 +1399,7 @@ static TCGReg tcg_out_tlb_read(TCGContext *s, TCGMemOp opc,
>      int add_off = offsetof(CPUArchState, tlb_table[mem_index][0].addend);
>      TCGReg base = TCG_AREG0;
>      TCGMemOp s_bits = opc & MO_SIZE;
> +    int a_bits = get_alignment_bits(opc);
>  
>      /* Extract the page index, shifted into place for tlb index.  */
>      if (TCG_TARGET_REG_BITS == 64) {
> @@ -1456,14 +1457,21 @@ static TCGReg tcg_out_tlb_read(TCGContext *s, TCGMemOp opc,
>           * the bottom bits and thus trigger a comparison failure on
>           * unaligned accesses
>           */
> +        if (a_bits > 0) {
> +            tcg_debug_assert((((1 << a_bits) - 1) & TLB_FLAGS_MASK) == 0);
> +        } else {
> +            a_bits = s_bits;
> +        }
>          tcg_out_rlw(s, RLWINM, TCG_REG_R0, addrlo, 0,
> +                    (32 - a_bits) & 31, 31 - TARGET_PAGE_BITS);

ppc32 can certainly support over-alignment, just like every other target.  It's
just that there are some 32-bit parts that don't support unaligned accesses.


r~