From: Peter Maydell <peter.maydell@linaro.org>
To: Pavel Dovgaluk <Pavel.Dovgaluk@ispras.ru>
Cc: Paolo Bonzini <pbonzini@redhat.com>,
Richard Henderson <rth7680@gmail.com>,
Leon Alrae <leon.alrae@imgtec.com>,
QEMU Developers <qemu-devel@nongnu.org>,
Aurelien Jarno <aurelien@aurel32.net>
Subject: Re: [Qemu-devel] [PATCH v2 0/3] Fix exceptions handling for MIPS and i386
Date: Thu, 18 Jun 2015 08:51:10 +0100 [thread overview]
Message-ID: <CAFEAcA8Gp6f0bSS-MOgvdP2HcAWeGV7FeJOoM-u3jpcL-DTz9w@mail.gmail.com> (raw)
In-Reply-To: <55826f70.2215370a.4634.ffff91b2SMTPIN_ADDED_BROKEN@mx.google.com>
On 18 June 2015 at 08:12, Pavel Dovgaluk <Pavel.Dovgaluk@ispras.ru> wrote:
>> From: Aurelien Jarno [mailto:aurelien@aurel32.net]
>> Looking at how icount work, I see it's basically a variable in the CPU
>> state (icount_decr.u16.low), which is already accessed from the TB.
>> Couldn't we adjust it using additional code before generating an
>> exception, when in icount mode.
>>
>> For example for MIPS, we can add some code before generate_exception
>> which use the value from s->gen_opc_icount[j] to adjust
>> the variable icount_decr.u16.low.
>
> It is possible, but it will incur additional overhead, because we will
> have to update icount every time the exception might be generated.
> We'll have to update icount value before and after every helper call,
> that can cause an exception:
>
> icount -= n
> ...
> instr_k
> icount += n - k
> helper
> icount -= n - k
> ...
>
> And this overhead will slowdown the code even if no exception occur.
Right, this is a tradeoff: in some cases it's faster to assume
no exception and handle state resync by doing a retranslate.
In some cases it's faster to assume there will be an exception
and do a manual sync. Guest load/store is obviously in the
first category. Guest doing an instruction which always takes
an exception (like syscall insns) is in the second category.
For other cases there's a choice. We need to support both
approaches; obviously you can argue for any particular case
whether it should be approach 1 or approach 2.
thanks
-- PMM
next prev parent reply other threads:[~2015-06-18 7:51 UTC|newest]
Thread overview: 29+ messages / expand[flat|nested] mbox.gz Atom feed top
2015-06-17 12:41 [Qemu-devel] [PATCH v2 0/3] Fix exceptions handling for MIPS and i386 Pavel Dovgalyuk
2015-06-17 12:42 ` [Qemu-devel] [PATCH v2 1/3] softmmu: add helper function to pass through retaddr Pavel Dovgalyuk
2015-06-17 12:53 ` Paolo Bonzini
2015-06-18 5:17 ` Pavel Dovgaluk
2015-06-18 8:16 ` Paolo Bonzini
2015-06-18 8:20 ` Aurelien Jarno
2015-06-18 9:24 ` Pavel Dovgaluk
2015-06-18 9:30 ` Paolo Bonzini
2015-06-18 9:33 ` Pavel Dovgaluk
2015-06-18 9:35 ` Paolo Bonzini
2015-06-17 12:42 ` [Qemu-devel] [PATCH v2 2/3] target-mips: exceptions handling in icount mode Pavel Dovgalyuk
2015-06-17 13:05 ` Aurelien Jarno
2015-06-17 12:42 ` [Qemu-devel] [PATCH v2 3/3] target-i386: fix memory operations in helpers Pavel Dovgalyuk
2015-06-17 13:27 ` Aurelien Jarno
2015-06-17 13:24 ` [Qemu-devel] [PATCH v2 0/3] Fix exceptions handling for MIPS and i386 Aurelien Jarno
2015-06-18 6:18 ` Pavel Dovgaluk
2015-06-17 14:19 ` Aurelien Jarno
2015-06-18 7:12 ` Pavel Dovgaluk
2015-06-18 8:16 ` Aurelien Jarno
2015-06-18 8:58 ` Pavel Dovgaluk
2015-06-18 9:08 ` Aurelien Jarno
2015-06-18 9:29 ` Paolo Bonzini
2015-06-18 9:42 ` Aurelien Jarno
2015-06-18 10:02 ` Paolo Bonzini
2015-06-18 17:42 ` Aurelien Jarno
2015-06-19 5:09 ` Pavel Dovgaluk
2015-06-19 8:22 ` Aurelien Jarno
[not found] ` <55826f70.2215370a.4634.ffff91b2SMTPIN_ADDED_BROKEN@mx.google.com>
2015-06-18 7:51 ` Peter Maydell [this message]
2015-06-18 7:56 ` Pavel Dovgaluk
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