Re: [PATCH v3] target/s390x: Implement the MVPG condition-code-option bit

[David Hildenbrand <david@redhat.com>]

On 03.03.21 12:36, Thomas Huth wrote:
> On 03/03/2021 07.25, Thomas Huth wrote:
>> On 03/03/2021 03.44, Richard Henderson wrote:
>>> If the CCO bit is set, MVPG should not generate an exception but
>>> report page translation faults via a CC code.
>>>
>>> Create a new helper, access_prepare_nf, which can use probe_access_flags
>>> in non-faulting mode, and then handle watchpoints.
>>>
>>> Cc: David Hildenbrand <david@redhat.com>
>>> Reported-by: Thomas Huth <thuth@redhat.com>
>>> Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
>>> ---
>>>    target/s390x/mem_helper.c | 87 ++++++++++++++++++++++++++++-----------
>>>    1 file changed, 64 insertions(+), 23 deletions(-)
>>>
>>> diff --git a/target/s390x/mem_helper.c b/target/s390x/mem_helper.c
>>> index 25cfede806..b397333c0b 100644
>>> --- a/target/s390x/mem_helper.c
>>> +++ b/target/s390x/mem_helper.c
>>> @@ -130,28 +130,62 @@ typedef struct S390Access {
>>>        int mmu_idx;
>>>    } S390Access;
>>> +static bool access_prepare_nf(S390Access *access, CPUS390XState *env,
>>> +                              bool nofault, vaddr vaddr1, int size,
>>> +                              MMUAccessType access_type,
>>> +                              int mmu_idx, uintptr_t ra)
>>> +{
>>> +    void *haddr1, *haddr2 = NULL;
>>> +    int size1, size2;
>>> +    vaddr vaddr2 = 0;
>>> +    int flags;
>>> +
>>> +    assert(size > 0 && size <= 4096);
>>> +
>>> +    size1 = MIN(size, -(vaddr1 | TARGET_PAGE_MASK)),
>>> +    size2 = size - size1;
>>> +
>>> +    flags = probe_access_flags(env, vaddr1, access_type, mmu_idx,
>>> +                               nofault, &haddr1, ra);
>>> +    if (unlikely(size2)) {
>>> +        /* The access crosses page boundaries. */
>>> +        vaddr2 = wrap_address(env, vaddr1 + size1);
>>> +        flags |= probe_access_flags(env, vaddr2, access_type, mmu_idx,
>>> +                                    nofault, &haddr2, ra);
>>> +    }
>>> +
>>> +    if (unlikely(flags & TLB_INVALID_MASK)) {
>>> +        return false;
>>> +    }
>>> +    if (unlikely(flags & TLB_WATCHPOINT)) {
>>> +        /* S390 does not presently use transaction attributes. */
>>> +        cpu_check_watchpoint(env_cpu(env), vaddr1, size,
>>> +                             MEMTXATTRS_UNSPECIFIED,
>>> +                             (access_type == MMU_DATA_STORE
>>> +                              ? BP_MEM_WRITE : BP_MEM_READ), ra);
>>> +    }
>>> +
>>> +    *access = (S390Access) {
>>> +        .vaddr1 = vaddr1,
>>> +        .vaddr2 = vaddr2,
>>> +        .haddr1 = haddr1,
>>> +        .haddr2 = haddr2,
>>> +        .size1 = size1,
>>> +        .size2 = size2,
>>> +        .mmu_idx = mmu_idx
>>> +    };
>>> +    return true;
>>> +}
>>> +
>>>    static S390Access access_prepare(CPUS390XState *env, vaddr vaddr, int size,
>>>                                     MMUAccessType access_type, int mmu_idx,
>>>                                     uintptr_t ra)
>>>    {
>>> -    S390Access access = {
>>> -        .vaddr1 = vaddr,
>>> -        .size1 = MIN(size, -(vaddr | TARGET_PAGE_MASK)),
>>> -        .mmu_idx = mmu_idx,
>>> -    };
>>> -
>>> -    g_assert(size > 0 && size <= 4096);
>>> -    access.haddr1 = probe_access(env, access.vaddr1, access.size1,
>>> access_type,
>>> -                                 mmu_idx, ra);
>>> -
>>> -    if (unlikely(access.size1 != size)) {
>>> -        /* The access crosses page boundaries. */
>>> -        access.vaddr2 = wrap_address(env, vaddr + access.size1);
>>> -        access.size2 = size - access.size1;
>>> -        access.haddr2 = probe_access(env, access.vaddr2, access.size2,
>>> -                                     access_type, mmu_idx, ra);
>>> -    }
>>> -    return access;
>>> +    S390Access ret;
>>> +    bool ok = access_prepare_nf(&ret, env, false, vaddr, size,
>>> +                                access_type, mmu_idx, ra);
>>> +    assert(ok);
>>> +    return ret;
>>>    }
>>>    /* Helper to handle memset on a single page. */
>>> @@ -845,8 +879,10 @@ uint32_t HELPER(mvpg)(CPUS390XState *env, uint64_t
>>> r0, uint64_t r1, uint64_t r2)
>>>        const int mmu_idx = cpu_mmu_index(env, false);
>>>        const bool f = extract64(r0, 11, 1);
>>>        const bool s = extract64(r0, 10, 1);
>>> +    const bool cco = extract64(r0, 8, 1);
>>>        uintptr_t ra = GETPC();
>>>        S390Access srca, desta;
>>> +    bool ok;
>>>        if ((f && s) || extract64(r0, 12, 4)) {
>>>            tcg_s390_program_interrupt(env, PGM_SPECIFICATION, GETPC());
>>> @@ -858,13 +894,18 @@ uint32_t HELPER(mvpg)(CPUS390XState *env, uint64_t
>>> r0, uint64_t r1, uint64_t r2)
>>>        /*
>>>         * TODO:
>>>         * - Access key handling
>>> -     * - CC-option with surpression of page-translation exceptions
>>>         * - Store r1/r2 register identifiers at real location 162
>>>         */
>>> -    srca = access_prepare(env, r2, TARGET_PAGE_SIZE, MMU_DATA_LOAD, mmu_idx,
>>> -                          ra);
>>> -    desta = access_prepare(env, r1, TARGET_PAGE_SIZE, MMU_DATA_STORE,
>>> mmu_idx,
>>> -                           ra);
>>> +    ok = access_prepare_nf(&srca, env, cco, r2, TARGET_PAGE_SIZE,
>>> +                           MMU_DATA_LOAD, mmu_idx, ra);
>>> +    if (!ok) {
>>> +        return 2;
>>> +    }
>>> +    ok = access_prepare_nf(&desta, env, cco, r1, TARGET_PAGE_SIZE,
>>> +                           MMU_DATA_STORE, mmu_idx, ra);
>>> +    if (!ok) {
>>> +        return 1;
>>> +    }
>>
>> Thanks, this looks promising, but one of the MVPG kvm-unit-tests is still
>> failing with this patch - the one that checks for an exception if the
>> destination page is marked as read-only. MVPG only returns CC1 for invalid
>> page table entries - but if the page is write-protected, it still causes a
>> protection exception. That's why I've been checking "if (exc && exc !=
>> PGM_PROTECTION)" in my version of the patch.
> 
> FWIW, I can get the MVPG kvm-unit-test working with your patch if I add
> this on top:
> 
> diff a/target/s390x/mem_helper.c b/target/s390x/mem_helper.c
> --- a/target/s390x/mem_helper.c
> +++ b/target/s390x/mem_helper.c
> @@ -904,7 +904,14 @@ uint32_t HELPER(mvpg)(CPUS390XState *env, uint64_t r0, uint64_t r1, uint64_t r2)
>        ok = access_prepare_nf(&desta, env, cco, r1, TARGET_PAGE_SIZE,
>                               MMU_DATA_STORE, mmu_idx, ra);
>        if (!ok) {
> -        return 1;
> +        ok = access_prepare_nf(&desta, env, cco, r1, TARGET_PAGE_SIZE,
> +                               MMU_DATA_LOAD, mmu_idx, ra);
> +        if (!ok) {
> +            return 1;
> +        }
> +        /* If reading was ok, then the page must be protected. */
> +        /* TODO: Set a translation exception code in lowcore? */
> +        tcg_s390_program_interrupt(env, PGM_PROTECTION, ra);
>        }
>        access_memmove(env, &desta, &srca, ra);
>        return 0; /* data moved */
> 
> ... yeah, it's ugly to call access_prepare_nf() again with MMU_DATA_LOAD,
> and it's still missing the translation exception code ... but at least
> the kvm-unit-test is happy that way...

As I said, can't we store the last exception we had during tlb_fill and 
use that in case returns access_prepare_nf() returns an error to 
identify the actual exception?

-- 
Thanks,

David / dhildenb