Re: [Qemu-devel] [PATCH-for-4.2 v1 1/9] s390x/mmu: Better ASC selection in s390_cpu_get_phys_page_debug()

[David Hildenbrand <david@redhat.com>]

On 12.08.19 15:40, Cornelia Huck wrote:
> On Mon, 12 Aug 2019 09:52:56 +0200
> David Hildenbrand <david@redhat.com> wrote:
> 
>> On 12.08.19 09:12, Thomas Huth wrote:
>>> On 8/5/19 5:29 PM, David Hildenbrand wrote:  
>>>> Let's select the ASC before calling the function and use MMU_DATA_LOAD.
>>>> This is a preparation to:
>>>> - Remove the ASC magic depending on the access mode from mmu_translate
>>>> - Implement IEP support, where we could run into access exceptions
>>>>   trying to fetch instructions
>>>>
>>>> Signed-off-by: David Hildenbrand <david@redhat.com>
>>>> ---
>>>>  target/s390x/helper.c | 10 +++++++++-
>>>>  1 file changed, 9 insertions(+), 1 deletion(-)
>>>>
>>>> diff --git a/target/s390x/helper.c b/target/s390x/helper.c
>>>> index 13ae9909ad..08166558a0 100644
>>>> --- a/target/s390x/helper.c
>>>> +++ b/target/s390x/helper.c
>>>> @@ -58,7 +58,15 @@ hwaddr s390_cpu_get_phys_page_debug(CPUState *cs, vaddr vaddr)
>>>>          vaddr &= 0x7fffffff;
>>>>      }
>>>>  
>>>> -    if (mmu_translate(env, vaddr, MMU_INST_FETCH, asc, &raddr, &prot, false)) {
>>>> +    /*
>>>> +     * We want to read the code, however, not run into access exceptions  
>>>
>>> Is this really a safe assumption here that we always use this to
>>> translate code addresses and not data addresses? ... I don't think so.
>>> For example with the "gva2gpa" HMP command, I'd rather expect that it
>>> also works with the secondary space mode...?  
>>
>> Well, it's what current code does. I am not changing that behavior.
> 
> Agreed, that is not actively breaking something.
> 
>>
>> While it is in general broken to have a single interface to debug
>> code+data (which is only a problem on s390x), it makes a lot of sense if
>> you think about single-stepping through disassembled code using the
>> gdbstub. Or dumping code where you crashed.
> 
> What about the memsave interface?

I guess the same problem:

"save to disk virtual memory dump starting at @var{addr} of size
@var{size}" -  which virtual memory (code vs. data)? These old interface
are really x86 specific (meaning: it made sense this way for x86)

I'd like to note that if our KVM guest is in AR mode, we would now no
longer be able to crash it :) (well, a nice side-effect of instruction
fetches not going via AR mode).

-- 

Thanks,

David / dhildenb