From: Peter Zijlstra <peterz@infradead.org>
To: Paolo Bonzini <pbonzini@redhat.com>
Cc: Sean Christopherson <seanjc@google.com>,
x86@kernel.org, linux-kernel@vger.kernel.org,
jpoimboe@redhat.com, mark.rutland@arm.com, dvyukov@google.com,
mbenes@suse.cz
Subject: Re: [RFC][PATCH 15/22] x86,vmx: Remove .fixup usage
Date: Sat, 6 Nov 2021 09:36:46 +0100 [thread overview]
Message-ID: <20211106083646.GT174703@worktop.programming.kicks-ass.net> (raw)
In-Reply-To: <18358428-e0cd-d329-7558-6eb548b3fc53@redhat.com>
On Sat, Nov 06, 2021 at 08:05:51AM +0100, Paolo Bonzini wrote:
> On 11/5/21 19:17, Sean Christopherson wrote:
> > On Thu, Nov 04, 2021, Peter Zijlstra wrote:
> > > In the vmread exceptin path, use the, thus far, unused output register
> > > to push the @fault argument onto the stack. This, in turn, enables the
> > > exception handler to not do pushes and only modify that register when
> > > an exception does occur.
> > >
> > > Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
> > > ---
> > > arch/x86/kvm/vmx/vmx_ops.h | 14 ++++++--------
> > > 1 file changed, 6 insertions(+), 8 deletions(-)
> > >
> > > --- a/arch/x86/kvm/vmx/vmx_ops.h
> > > +++ b/arch/x86/kvm/vmx/vmx_ops.h
> > > @@ -80,9 +80,11 @@ static __always_inline unsigned long __v
> > > * @field, and bounce through the trampoline to preserve
> > > * volatile registers.
> > > */
> > > - "push $0\n\t"
> > > + "xorl %k1, %k1\n\t"
> > > + "2:\n\t"
> > > + "push %1\n\t"
> > > "push %2\n\t"
> >
> > This trick doesn't work if the compiler selects the same GPR for %1 and %2, as
> > the "field" will get lost.
>
> Ouch, good catch. It should be actually very simple to fix it, just mark
> "value" as an "early clobber" output:
>
> : ASM_CALL_CONSTRAINT, "=&r"(value) : "r"(field) : "cc");
>
> That's an output which is written before the instruction is finished using
> the input operands. The manual even says "this operand may not lie in a
> register that is read by the instruction or as part of any memory address",
> which is exactly what you caught with %1 and %2 both being the same GPR.
Yes, but as Sean points out, that will negatively affect code-gen on the
happy path. But perhaps that's acceptable if we add the asm-goto-output
variant?
next prev parent reply other threads:[~2021-11-06 8:50 UTC|newest]
Thread overview: 59+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-11-04 16:47 [RFC][PATCH 00/22] x86: Remove anonymous out-of-line fixups Peter Zijlstra
2021-11-04 16:47 ` [RFC][PATCH 01/22] bitfield.h: Fix "type of reg too small for mask" test Peter Zijlstra
2021-11-04 16:47 ` [RFC][PATCH 02/22] x86,mmx_32: Remove .fixup usage Peter Zijlstra
2021-11-04 18:00 ` Borislav Petkov
2021-11-05 11:20 ` David Laight
2021-11-04 20:22 ` Josh Poimboeuf
2021-11-05 8:05 ` Peter Zijlstra
2021-11-04 16:47 ` [RFC][PATCH 03/22] x86,copy_user_64: " Peter Zijlstra
2021-11-04 16:47 ` [RFC][PATCH 04/22] x86,copy_mc_64: " Peter Zijlstra
2021-11-04 16:47 ` [RFC][PATCH 05/22] x86,entry_64: " Peter Zijlstra
2021-11-04 16:47 ` [RFC][PATCH 06/22] x86,entry_32: " Peter Zijlstra
2021-11-04 20:39 ` Josh Poimboeuf
2021-11-05 7:43 ` Peter Zijlstra
2021-11-04 16:47 ` [RFC][PATCH 07/22] x86,extable: Extend extable functionality Peter Zijlstra
2021-11-04 21:49 ` Josh Poimboeuf
2021-11-05 7:54 ` Peter Zijlstra
2021-11-05 10:16 ` Mark Rutland
2021-11-05 17:32 ` Sean Christopherson
2021-11-05 18:45 ` Peter Zijlstra
2021-11-05 19:17 ` Sean Christopherson
2021-11-05 19:32 ` Peter Zijlstra
2021-11-05 19:47 ` Sean Christopherson
2021-11-05 20:15 ` Peter Zijlstra
2021-11-05 20:26 ` Peter Zijlstra
2021-11-05 22:30 ` Sean Christopherson
2021-11-04 16:47 ` [RFC][PATCH 08/22] x86,msr: Remove .fixup usage Peter Zijlstra
2021-11-04 16:47 ` [RFC][PATCH 09/22] x86,futex: " Peter Zijlstra
2021-11-04 16:47 ` [RFC][PATCH 10/22] x86,uaccess: " Peter Zijlstra
2021-11-04 22:28 ` Josh Poimboeuf
2021-11-04 16:47 ` [RFC][PATCH 11/22] x86,xen: " Peter Zijlstra
2021-11-04 22:31 ` Josh Poimboeuf
2021-11-05 7:56 ` Peter Zijlstra
2021-11-04 16:47 ` [RFC][PATCH 12/22] x86,fpu: " Peter Zijlstra
2021-11-04 16:47 ` [RFC][PATCH 13/22] x86,segment: " Peter Zijlstra
2021-11-04 16:47 ` [RFC][PATCH 14/22] x86,ftrace: " Peter Zijlstra
2021-11-04 22:35 ` Josh Poimboeuf
2021-11-05 7:57 ` Peter Zijlstra
2021-11-04 16:47 ` [RFC][PATCH 15/22] x86,vmx: " Peter Zijlstra
2021-11-04 18:50 ` Paolo Bonzini
2021-11-05 18:17 ` Sean Christopherson
2021-11-05 18:52 ` Peter Zijlstra
2021-11-05 20:58 ` Peter Zijlstra
2021-11-05 22:29 ` Sean Christopherson
2021-11-06 7:05 ` Paolo Bonzini
2021-11-06 8:36 ` Peter Zijlstra [this message]
2021-11-07 19:13 ` Paolo Bonzini
2021-11-06 8:28 ` Peter Zijlstra
2021-11-04 16:47 ` [RFC][PATCH 16/22] x86,checksum_32: " Peter Zijlstra
2021-11-04 16:47 ` [RFC][PATCH 17/22] x86,sgx: " Peter Zijlstra
2021-11-04 16:47 ` [RFC][PATCH 18/22] x86,kvm: " Peter Zijlstra
2021-11-04 18:50 ` Paolo Bonzini
2021-11-05 7:58 ` Peter Zijlstra
2021-11-04 16:47 ` [RFC][PATCH 19/22] x86,usercopy_32: Simplify Peter Zijlstra
2021-11-04 16:47 ` [RFC][PATCH 20/22] x86,usercopy: Remove .fixup usage Peter Zijlstra
2021-11-04 16:47 ` [RFC][PATCH 21/22] x86,word-at-a-time: " Peter Zijlstra
2021-11-04 23:33 ` Josh Poimboeuf
2021-11-05 8:04 ` Peter Zijlstra
2021-11-04 16:47 ` [RFC][PATCH 22/22] x86: Remove .fixup section Peter Zijlstra
2021-11-04 23:00 ` Josh Poimboeuf
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