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Re: [qemu-s390x] [Qemu-devel] [PATCH v1 2/4] s390x/tcg: Introduce probe_

From: Richard Henderson
Subject: Re: [qemu-s390x] [Qemu-devel] [PATCH v1 2/4] s390x/tcg: Introduce probe_read_access()
Date: Wed, 21 Aug 2019 15:43:23 -0700
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.8.0

On 8/21/19 3:31 PM, Richard Henderson wrote:
>> Yes, that's what I mean, TARGET_PAGE_SIZE, but eventually crossing a
>> page boundary. The longer I stare at the MVCL code, the more broken it
>> is. There are more nice things buried in the PoP. MVCL does not detect
>> access exceptions beyond the next 2k. So we have to limit it there
>> differently.
> That language is indeed odd.
> The only reading of that paragraph that makes sense to me is that the hardware
> *must* interrupt MVCL after every 2k bytes processed.  The idea that the user
> can magically write to a read-only page simply by providing length = 2MB and
> page that is initially writable is dumb.  I cannot imagine that is a correct
> reading.
> Getting clarification from an IBM engineer on that would be good; otherwise I
> would just ignore that and proceed as if all access checks are performed.

FWIW, splitting the operation at every aligned 2k boundary is exactly what the
Hercules emulator does:

    len3 = NOCROSS2KL(addr1,len1) ? len1 : (int)(0x800 - (addr1 & 0x7FF));
    len4 = NOCROSS2KL(addr2,len2) ? len2 : (int)(0x800 - (addr2 & 0x7FF));
    len = len3 < len4 ? len3 : len4;
    /* Use concpy to ensure Concurrent block update consistency */
    concpy (regs, dest, source, len);

After this it writes back the lengths and addresses to the
register file, and then if necessary loops back to the address
translation step.


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