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Re: [Qemu-devel] [PATCH 06/11] target/arm: Reset btype for direct branch

From: Richard Henderson
Subject: Re: [Qemu-devel] [PATCH 06/11] target/arm: Reset btype for direct branches and syscalls
Date: Tue, 29 Jan 2019 06:05:27 -0800
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.4.0

On 1/29/19 1:57 AM, Peter Maydell wrote:
>>> The advantage of picking the other choice (SPSR_ELx.BTYPE ==
>>> PSTATE.BTYPE) is that it means that the behaviour is identical
>>> for all exceptions (async or sync of any type) and we don't
>>> do the work of clearing the BTYPE field (which will happen
>>> potentially in "normal" guest code if we're not in a guarded page,
>>> I think).
>> Well, BTYPE is in the TB flags, so we know it's already zero in that case, so
>> there's no extra work.
> It's not zero if we just did a BR Xn to get to this SVC insn, is it?

I guess I misunderstood what you meant by "extra" work.
It's not "extra" if btype is known to not be zero...

Anyway, in v2 the clearing of btype happens in cpu_loop,
more like what the kernel would have to do.


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