From: Sean Christopherson <seanjc@google.com>
To: Jacob Xu <jacobhxu@google.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>,
Jim Mattson <jmattson@google.com>, kvm list <kvm@vger.kernel.org>
Subject: Re: [kvm-unit-tests PATCH v2] x86: Do not assign values to unaligned pointer to 128 bits
Date: Thu, 6 May 2021 19:25:26 +0000 [thread overview]
Message-ID: <YJRCpv9O/Q24DKmZ@google.com> (raw)
In-Reply-To: <CAJ5mJ6gYmwXEQZASk8A_Ozt6asW6ZDTnDs83nCfLNTa62x7n+g@mail.gmail.com>
On Thu, May 06, 2021, Jacob Xu wrote:
> > memset() takes a void *, which it casts to an char, i.e. it works on one byte at
> a time.
> Huh, TIL. Based on this I'd thought that I don't need a cast at all,
> but doing so actually results in a movaps instruction.
Ewwww. That's likely because emulator.c does:
#define memset __builtin_memset
and the compiler is clever enough to know that __attribute__((vector_size(16)))
means the variable is (supposed to be) aligned.
> I've changed the cast back to (uint8_t *).
I assume removing the above #define and grabbing memset() from string.c fixes
the movaps generation? If so, that has my vote, as opposed to fudging around
the compiler by casting to uint8_t *.
As evidenced by this issue, using the compiler's memset() in kvm-unit-tests seems
inherently dangerous since the tests are often doing intentionally stupid things.
next prev parent reply other threads:[~2021-05-06 19:25 UTC|newest]
Thread overview: 6+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-05-06 18:49 [kvm-unit-tests PATCH v2] x86: Do not assign values to unaligned pointer to 128 bits Jacob Xu
2021-05-06 18:57 ` Sean Christopherson
2021-05-06 19:13 ` Jacob Xu
2021-05-06 19:25 ` Sean Christopherson [this message]
2021-05-06 20:11 ` Jim Mattson
2021-05-11 1:47 ` Jacob Xu
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