Re: [Qemu-devel] [PATCH v1 2/4] s390x/tcg: Introduce probe_read_access()

[David Hildenbrand]

On 21.08.19 19:26, Richard Henderson wrote:
> On 8/21/19 2:22 AM, David Hildenbrand wrote:
>> +/*
>> + * Make sure the read access is permitted and TLB entries are created. In
>> + * very rare cases it might happen that the actual accesses might need
>> + * new MMU translations. If the page tables were changed in between, we
>> + * might still trigger a fault. However, this seems to barely happen, so we
>> + * can ignore this for now.
>> + */
>> +void probe_read_access(CPUS390XState *env, uint64_t addr, uint64_t len,
>> +                       uintptr_t ra)
>> +{
>> +#ifdef CONFIG_USER_ONLY
>> +    if (!guest_addr_valid(addr) || !guest_addr_valid(addr + len - 1) ||
>> +        page_check_range(addr, len, PAGE_READ) < 0) {
>> +        s390_program_interrupt(env, PGM_ADDRESSING, ILEN_AUTO, ra);
>> +    }
>> +#else
>> +    while (len) {
>> +        const uint64_t pagelen = -(addr | -TARGET_PAGE_MASK);
>> +        const uint64_t curlen = MIN(pagelen, len);
>> +
>> +        cpu_ldub_data_ra(env, addr, ra);
>> +        addr = wrap_address(env, addr + curlen);
>> +        len -= curlen;
>> +    }
>> +#endif
>> +}
> 
> I don't think this is really the right approach, precisely because of the
> comment above.
> 
> I think we should
> 
> (1) Modify the generic probe_write to return the host address,
>     akin to tlb_vaddr_to_host except it *will* fault.
> 
> (2) Create a generic version of probe_write for CONFIG_USER_ONLY,
>     much like the one you have done for target/s390x.
> 
> (3) Create generic version of probe_read that does the same.
> 
> (4) Rewrite fast_memset and fast_memmove to fetch all of the host
>     addresses before doing any modifications.  The functions are
>     currently written as if len can be very large, handling any
>     number of pages.  Except that's not true.  While there are
>     several kinds of users apart from MVC, two pages are sufficient
>     for all users.
> 
>     Well, should be.  We would need to adjust do_mvcl to limit the
>     operation to TARGET_PAGE_SIZE (CC=3, cpu-determined number of
>     bytes moved without reaching end of first operand).
>     Which is probably a good idea anyway.  System mode should not
>     spend forever executing one instruction, as it would if you
>     pass in a 64-bit length from MVCLE.
> 

Related to that: yes, that's what I mentioned in the cover letter, the
MOVE variants are full of issues. MVCLE should be limited to 4096 bytes,
and we should return cc=3. However, MVCL (which also uses do_mvcl) is
also semi-broken (it's an interruptible instruction - not cc=3, we have
to update the register step by step). MVCL should return cc=3 in case
it's an destructive copy - also not implemented properly. mem_helpers.c
is full of issues.

-- 

Thanks,

David / dhildenb