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

[Cornelia Huck <cohuck@redhat.com>]

On Mon, 12 Aug 2019 15:45:25 +0200
David Hildenbrand <david@redhat.com> wrote:

> 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)

So, the general verdict is "gnarly interface, but at least not broken
for Linux guests", I guess.

> 
> 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).

Heh :)

Q: What do we need to consider beyond Linux guests? Probably not much,
given that they would be broken already...