Re: [kvm-unit-tests GIT PULL v2 11/14] s390x: Add tests for execute-type instructions

Nina Schoetterl-Glausch
2023-04-04T15:33:36Z

On Tue, 2023-04-04 at 17:05 +0200, Thomas Huth wrote:
> On 04/04/2023 16.54, Nina Schoetterl-Glausch wrote:
> > On Tue, 2023-04-04 at 16:15 +0200, Thomas Huth wrote:
> > > On 04/04/2023 13.36, Nico Boehr wrote:
> > > > From: Nina Schoetterl-Glausch <nsg@linux.ibm.com>
> > > >
> > > > Test the instruction address used by targets of an execute instruction.
> > > > When the target instruction calculates a relative address, the result is
> > > > relative to the target instruction, not the execute instruction.
> > > >
> > > > Reviewed-by: Janosch Frank <frankja@linux.ibm.com>
> > > > Signed-off-by: Nina Schoetterl-Glausch <nsg@linux.ibm.com>
> > > > Reviewed-by: Claudio Imbrenda <imbrenda@linux.ibm.com>
> > > > Link: https://lore.kernel.org/r/20230317112339.774659-1-nsg@linux.ibm.com
> > > > Signed-off-by: Nico Boehr <nrb@linux.ibm.com>
> > > > ---
> > > > s390x/Makefile | 1 +
> > > > s390x/ex.c | 188 ++++++++++++++++++++++++++++++++++++++++++++
> > > > s390x/unittests.cfg | 3 +
> > > > .gitlab-ci.yml | 1 +
> > > > 4 files changed, 193 insertions(+)
> > > > create mode 100644 s390x/ex.c
> > > >
> > > > diff --git a/s390x/Makefile b/s390x/Makefile
> > > > index ab146eb..a80db53 100644
> > > > --- a/s390x/Makefile
> > > > +++ b/s390x/Makefile
> > > > @@ -39,6 +39,7 @@ tests += $(TEST_DIR)/panic-loop-extint.elf
> > > > tests += $(TEST_DIR)/panic-loop-pgm.elf
> > > > tests += $(TEST_DIR)/migration-sck.elf
> > > > tests += $(TEST_DIR)/exittime.elf
> > > > +tests += $(TEST_DIR)/ex.elf
> > > >
> > > > pv-tests += $(TEST_DIR)/pv-diags.elf
> > > >
> > > > diff --git a/s390x/ex.c b/s390x/ex.c
> > > > new file mode 100644
> > > > index 0000000..dbd8030
> > > > --- /dev/null
> > > > +++ b/s390x/ex.c
> > > > @@ -0,0 +1,188 @@
> > > > +// SPDX-License-Identifier: GPL-2.0-only
> > > > +/*
> > > > + * Copyright IBM Corp. 2023
> > > > + *
> > > > + * Test EXECUTE (RELATIVE LONG).
> > > > + * These instructions execute a target instruction. The target instruction is formed
> > > > + * by reading an instruction from memory and optionally modifying some of its bits.
> > > > + * The execution of the target instruction is the same as if it was executed
> > > > + * normally as part of the instruction sequence, except for the instruction
> > > > + * address and the instruction-length code.
> > > > + */
> > > > +
> > > > +#include <libcflat.h>
> > > > +
> > > > +/*
> > > > + * Accesses to the operand of execute-type instructions are instruction fetches.
> > > > + * Minimum alignment is two, since the relative offset is specified by number of halfwords.
> > > > + */
> > > > +asm ( ".pushsection .text.exrl_targets,\"x\"\n"
> > > > +" .balign 2\n"
> > > > +" .popsection\n"
> > > > +);
> > > > +
> > > > +/*
> > > > + * BRANCH AND SAVE, register register variant.
> > > > + * Saves the next instruction address (address from PSW + length of instruction)
> > > > + * to the first register. No branch is taken in this test, because 0 is
> > > > + * specified as target.
> > > > + * BASR does *not* perform a relative address calculation with an intermediate.
> > > > + */
> > > > +static void test_basr(void)
> > > > +{
> > > > + uint64_t ret_addr, after_ex;
> > > > +
> > > > + report_prefix_push("BASR");
> > > > + asm volatile ( ".pushsection .text.exrl_targets\n"
> > > > + "0: basr %[ret_addr],0\n"
> > > > + " .popsection\n"
> > > > +
> > > > + " larl %[after_ex],1f\n"
> > > > + " exrl 0,0b\n"
> > > > + "1:\n"
> > > > + : [ret_addr] "=d" (ret_addr),
> > > > + [after_ex] "=d" (after_ex)
> > > > + );
> > > > +
> > > > + report(ret_addr == after_ex, "return address after EX");
> > > > + report_prefix_pop();
> > > > +}
> > > > +
> > > > +/*
> > > > + * BRANCH RELATIVE AND SAVE.
> > > > + * According to PoP (Branch-Address Generation), the address calculated relative
> > > > + * to the instruction address is relative to BRAS when it is the target of an
> > > > + * execute-type instruction, not relative to the execute-type instruction.
> > > > + */
> > > > +static void test_bras(void)
> > > > +{
> > > > + uint64_t after_target, ret_addr, after_ex, branch_addr;
> > > > +
> > > > + report_prefix_push("BRAS");
> > > > + asm volatile ( ".pushsection .text.exrl_targets\n"
> > > > + "0: bras %[ret_addr],1f\n"
> > > > + " nopr %%r7\n"
> > > > + "1: larl %[branch_addr],0\n"
> > > > + " j 4f\n"
> > > > + " .popsection\n"
> > > > +
> > > > + " larl %[after_target],1b\n"
> > > > + " larl %[after_ex],3f\n"
> > > > + "2: exrl 0,0b\n"
> > > > +/*
> > > > + * In case the address calculation is correct, we jump by the relative offset 1b-0b from 0b to 1b.
> > > > + * In case the address calculation is relative to the exrl (i.e. a test failure),
> > > > + * put a valid instruction at the same relative offset from the exrl, so the test continues in a
> > > > + * controlled manner.
> > > > + */
> > > > + "3: larl %[branch_addr],0\n"
> > > > + "4:\n"
> > > > +
> > > > + " .if (1b - 0b) != (3b - 2b)\n"
> > > > + " .error \"right and wrong target must have same offset\"\n"
> > > > + " .endif\n"
> > >
> > > FWIW, this is failing with Clang 15 for me:
> > >
> > > s390x/ex.c:81:4: error: expected absolute expression
> > > " .if (1b - 0b) != (3b - 2b)\n"
> > > ^
> > > <inline asm>:12:6: note: instantiated into assembly here
> > > .if (1b - 0b) != (3b - 2b)
> >
> > Seems gcc is smarter here than clang.
>
> Yeah, the assembler from clang is quite a bit behind on s390x ... in the
> past I was only able to compile the k-u-t with Clang when using the
> "-no-integrated-as" option ... but at least in the most recent version it
> seems to have caught up now enough to be very close to compile it with the
> built-in assembler, so it would be great to get this problem here fixed
> somehow, too...
>
> > Just deleting that .if block would work, it's basically only a static assert.
> > What do you think?
> > Other than that I can't think of anything.
>
> Yes, either delete it ... or maybe you could return the two values (1b - 0b)
> and (3b - 2b) as output from the asm statement and do an assert() in C instead?

No, that's too late, it'd crash before if the invariant doesn't hold.
Could do a runtime check in asm but I don't think it's worth it. So lets go for deletion.

Do you wan't to fix it up when pulling or do you want a new version and pull request?
>
> Thomas
>