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Re: [qemu-s390x] [PATCH-for-4.2 v1 1/9] s390x/mmu: Better ASC selection

From: David Hildenbrand
Subject: Re: [qemu-s390x] [PATCH-for-4.2 v1 1/9] s390x/mmu: Better ASC selection in s390_cpu_get_phys_page_debug()
Date: Mon, 12 Aug 2019 09:52:56 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.7.2

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 <address@hidden>
>> ---
>>  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.

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.

In Linux, code+data will luckily usually have the same virtual->physical
tables, so it's not a real issue.

> So maybe we need a proper MemTxAttrs bit or something similar for
> distinguishing instruction accesses from data accesses here?

There would first have to be a way to ask "get_phys_page_debug" to get
code or data for this to make sense. Right now we used it to get code.




David / dhildenb

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