From: Paolo Bonzini <pbonzini@redhat.com>
To: Bandan Das <bsd@redhat.com>
Cc: linux-kernel@vger.kernel.org, kvm@vger.kernel.org
Subject: Re: [RFC PATCH 3/4] KVM: emulate: avoid per-byte copying in instruction fetches
Date: Wed, 07 May 2014 10:40:07 +0200 [thread overview]
Message-ID: <5369F167.5000005@redhat.com> (raw)
In-Reply-To: <jpgbnvai4ry.fsf@nelium.bos.redhat.com>
Il 07/05/2014 06:36, Bandan Das ha scritto:
>> > + _x = *(_type __aligned(1) *) &_fc->data[ctxt->_eip - _fc->start]; \
> For my own understanding, how does the __aligned help here ?
Except for 16-byte SSE accesses, x86 doesn't distinguish aligned and
unaligned accesses. You can read 4 bytes at 0x2345 and the processor
will do the right thing. Still it's better to tell the compiler what
we're doing.
> Wouldn't
> that result in unaligned accesses that will actually impact performance ?
These accesses *can* and will in fact be unaligned. For example, say
you have "mov ax,0x1234" which is 0xb8 0x34 0x12. When you read it into
the fetch cache, you will have data[0] == 0xb8, data[1] == 0x34, data[2]
== 0x12. Fetching the 16-bit immediate from data[1] will then be an
unaligned access.
Paolo
next prev parent reply other threads:[~2014-05-07 8:40 UTC|newest]
Thread overview: 11+ messages / expand[flat|nested] mbox.gz Atom feed top
2014-05-06 18:16 [RFC PATCH 0/4] KVM: emulator: speed up instruction fetch Paolo Bonzini
2014-05-06 18:16 ` [RFC PATCH 1/4] KVM: emulate: speed up do_insn_fetch Paolo Bonzini
2014-05-07 2:30 ` Bandan Das
2014-05-07 8:32 ` Paolo Bonzini
2014-05-06 18:16 ` [RFC PATCH 2/4] KVM: emulate: avoid repeated calls to do_insn_fetch_bytes Paolo Bonzini
2014-05-07 4:21 ` Bandan Das
2014-05-07 8:34 ` Paolo Bonzini
2014-05-06 18:16 ` [RFC PATCH 3/4] KVM: emulate: avoid per-byte copying in instruction fetches Paolo Bonzini
2014-05-07 4:36 ` Bandan Das
2014-05-07 8:40 ` Paolo Bonzini [this message]
2014-05-06 18:16 ` [RFC PATCH 4/4] KVM: emulate: put pointers in the fetch_cache Paolo Bonzini
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