On 2/5/22 06:07, Peter Maydell wrote:
+ /*
+ * Overalignment: When we're asking for really large alignment,
+ * the actual access is always done above and all we need to do
+ * here is invoke the handler for SIGBUS.
+ */
I thought the access was in an annulled delay slot and so won't
be "done above" ?
Bad wording, I suppose. If the alignment check succeeds, then access is done above. If
the alignment check fails, there is no access to be performed, only to report failure.
+ switch ((unsigned)memop) {
+ case MO_BEUW | MO_UNALN:
+ case MO_BESW | MO_UNALN:
+ case MO_BEUL | MO_ALIGN_2:
+ case MO_BESL | MO_ALIGN_2:
+ case MO_BEUQ | MO_ALIGN_4:
+ /* Two loads: shift and combine. */
+ tcg_out_ldst(s, TCG_REG_T2, TCG_REG_T1, 0,
+ qemu_ld_opc[a_bits | MO_BE | (memop & MO_SIGN)]);
+ tcg_out_ldst(s, data, TCG_REG_T1, 1 << a_bits,
+ qemu_ld_opc[a_bits | MO_BE]);
+ tcg_out_arithi(s, TCG_REG_T2, TCG_REG_T2, 8 << a_bits, SHIFT_SLLX);
Why are we calculating the offset in memory of the second half of
the data and the amount to shift it by using the alignment-bits
rather than the size-bits ? Because of the cases we know that
here a_bits == s_bits - 1, but I think it would be clearer to
work in terms of the size.
Ok.
+ tcg_out_arithi(s, TCG_REG_T1, TCG_REG_T1, 3, ARITH_ANDN);
+ tcg_out_ldst(s, TCG_REG_T2, TCG_REG_T1, 0, LDUW);
Doesn't this give the wrong fault-address value to the guest
(ie not the address it used for the load, but a rounded-down one)
if we take a SIGSEGV? Or do we fix that up elsewhere?
Oops, no. Perhaps a single byte load to the zero register would fix that, without having
to go to full load-by-parts.
+ case MO_BEUQ | MO_ALIGN_2:
+ /*
+ * An extra test to verify alignment 2 is 5 insns, which
+ * is more than we would save by using the slightly smaller
+ * unaligned sequence above.
+ */
+ tcg_out_ldst(s, data, TCG_REG_T1, 0, LDUH);
+ for (int i = 2; i < 8; i += 2) {
+ tcg_out_ldst(s, TCG_REG_T2, TCG_REG_T1, i, LDUW);
Isn't this loading 2 + 3 * 4 == 14 bytes?
Oops. Got confused with the qemu vs sparc "word" there for a moment.
r~
