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From: Richard Henderson <rth@twiddle.net>
To: Paolo Bonzini <pbonzini@redhat.com>
Cc: Peter Maydell <peter.maydell@linaro.org>,
	patches@linaro.org, Riku Voipio <riku.voipio@iki.fi>,
	qemu-devel@nongnu.org,
	Laurent Desnogues <laurent.desnogues@gmail.com>,
	Aurelien Jarno <aurelien@aurel32.net>
Subject: Re: [Qemu-devel] [PATCH 0/2] make guest-base support mandatory for TCG backends
Date: Tue, 09 Oct 2012 11:22:09 -0700	[thread overview]
Message-ID: <50746B51.4050105@twiddle.net> (raw)
In-Reply-To: <50743BAA.8050306@redhat.com>

On 10/09/2012 07:58 AM, Paolo Bonzini wrote:
> Perhaps you want to revive this patch:
> 
> http://lists.gnu.org/archive/html/qemu-devel/2010-06/msg00509.html

That needs careful benchmarking.

For i686 it's a small code size optimization.  It's unlikely to pay
off for guest_base.  A variant of this that sets the bounds of the
segment for -R size properly could be interesting for a different
reason: ensuring that the guest cannot access host memory.

I once had a patch that used %gs for i686 env (aka areg0).  With that
we get to free up a register for use by TCG, potentially mitigating
any slowdown from segmentation.

I suspect that the different semantics for 64-bit segments mean
that it's more likely to pay off there.  And when running on
very recent silicon we may not even need to rely on OS support.
See the wrfsbase insn that becomes available with Ivy Bridge.


r~

  reply	other threads:[~2012-10-09 18:22 UTC|newest]

Thread overview: 10+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2012-10-09 13:16 [Qemu-devel] [PATCH 0/2] make guest-base support mandatory for TCG backends Peter Maydell
2012-10-09 13:16 ` [Qemu-devel] [PATCH 1/2] configure: Remove unnecessary host_guest_base code Peter Maydell
2012-10-09 13:30   ` Richard Henderson
2012-10-09 13:16 ` [Qemu-devel] [PATCH 2/2] tcg: Remove TCG_TARGET_HAS_GUEST_BASE define Peter Maydell
2012-10-09 13:31   ` Richard Henderson
2012-10-09 13:28 ` [Qemu-devel] [PATCH 0/2] make guest-base support mandatory for TCG backends Richard Henderson
2012-10-09 14:50   ` Laurent Desnogues
2012-10-09 14:58     ` Paolo Bonzini
2012-10-09 18:22       ` Richard Henderson [this message]
2012-10-09 14:17 ` Aurelien Jarno

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