From: David Laight <david.laight.linux@gmail.com>
To: Brian Gerst <brgerst@gmail.com>
Cc: "H. Peter Anvin" <hpa@zytor.com>, Borislav Petkov <bp@alien8.de>,
Mauricio Faria de Oliveira <mfo@igalia.com>,
Thomas Gleixner <tglx@kernel.org>, Ingo Molnar <mingo@redhat.com>,
Dave Hansen <dave.hansen@linux.intel.com>,
x86@kernel.org, Juergen Gross <jgross@suse.com>,
Alexey Dobriyan <adobriyan@gmail.com>,
Boris Ostrovsky <boris.ostrovsky@oracle.com>,
kernel-dev@igalia.com, linux-kernel@vger.kernel.org,
xen-devel@lists.xenproject.org
Subject: Re: [PATCH v7 2/5] x86/asm: add volatile, clobbers and zero-length check in inline memcmp
Date: Sat, 25 Jul 2026 11:34:45 +0100 [thread overview]
Message-ID: <20260725113445.14ccf42f@pumpkin> (raw)
In-Reply-To: <CAMzpN2hdw60=Un+mAFS5CEQqDRZ-d3fMogEwE4NxUe75P-GfSA@mail.gmail.com>
On Thu, 23 Jul 2026 20:49:13 -0400
Brian Gerst <brgerst@gmail.com> wrote:
...
> I had originally thought that the test for RCX==0 was also at the end
> of the loop, after the decrement. But a closer look at the
> pseudo-code in the SDM does show that the test is at the start of the
> loop. Starting with RCX==0 does indeed skip the comparison and does
> not touch ZF. Your suggestion is good, but should have a comment
> explaining why it's there to someone reading the code in the future.
I have a feeling that was a change between the 8086 and 80286.
I don't have an 8086 book (any more) but I'm sure I remember fixing
bugs where %cx of zero caused 65536 iterations.
(Fun to find because the vector table is 'first in the line to be
overwritten.)
David
next prev parent reply other threads:[~2026-07-25 10:35 UTC|newest]
Thread overview: 27+ messages / expand[flat|nested] mbox.gz Atom feed top
2026-07-21 15:56 [PATCH v7 0/5] x86/pvh: fix unbootable VMs again (PVH + KASAN) Mauricio Faria de Oliveira
2026-07-21 15:56 ` [PATCH v7 1/5] x86/asm, x86/boot: expose inline memcmp Mauricio Faria de Oliveira
2026-07-21 15:56 ` [PATCH v7 2/5] x86/asm: add volatile, clobbers and zero-length check in " Mauricio Faria de Oliveira
2026-07-22 17:03 ` Borislav Petkov
2026-07-22 18:45 ` H. Peter Anvin
2026-07-23 6:59 ` Jan Beulich
2026-07-23 23:12 ` H. Peter Anvin
2026-07-24 0:35 ` Mauricio Faria de Oliveira
2026-07-24 21:53 ` H. Peter Anvin
2026-07-24 23:35 ` H. Peter Anvin
2026-07-24 23:57 ` H. Peter Anvin
2026-07-25 0:25 ` H. Peter Anvin
2026-07-25 10:27 ` David Laight
2026-07-23 19:37 ` Brian Gerst
2026-07-23 23:13 ` H. Peter Anvin
2026-07-24 0:49 ` Brian Gerst
2026-07-24 21:40 ` H. Peter Anvin
2026-07-25 10:34 ` David Laight [this message]
2026-07-23 23:20 ` Mauricio Faria de Oliveira
2026-07-22 18:48 ` H. Peter Anvin
2026-07-23 23:26 ` Mauricio Faria de Oliveira
2026-07-23 23:17 ` Mauricio Faria de Oliveira
2026-07-21 15:56 ` [PATCH v7 3/5] x86/asm: group inline string functions Mauricio Faria de Oliveira
2026-07-23 21:59 ` Borislav Petkov
2026-07-24 0:51 ` Mauricio Faria de Oliveira
2026-07-21 15:56 ` [PATCH v7 4/5] x86/cpuid: fix unbootable VMs by really inlining memcmp() in hypervisor_cpuid_base() Mauricio Faria de Oliveira
2026-07-21 15:56 ` [PATCH v7 5/5] x86/pvh: fix unbootable VMs by really inlining memset() in xen_prepare_pvh() Mauricio Faria de Oliveira
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