From: Pawan Gupta <pawan.kumar.gupta@linux.intel.com>
To: Andrew Cooper <andrew.cooper3@citrix.com>
Cc: Thomas Gleixner <tglx@linutronix.de>,
Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
Dave Hansen <dave.hansen@linux.intel.com>,
x86@kernel.org, "H. Peter Anvin" <hpa@zytor.com>,
Peter Zijlstra <peterz@infradead.org>,
Josh Poimboeuf <jpoimboe@kernel.org>,
Andy Lutomirski <luto@kernel.org>,
Jonathan Corbet <corbet@lwn.net>,
Sean Christopherson <seanjc@google.com>,
Paolo Bonzini <pbonzini@redhat.com>,
tony.luck@intel.com, ak@linux.intel.com,
tim.c.chen@linux.intel.com, linux-kernel@vger.kernel.org,
linux-doc@vger.kernel.org, kvm@vger.kernel.org,
Alyssa Milburn <alyssa.milburn@linux.intel.com>,
Daniel Sneddon <daniel.sneddon@linux.intel.com>,
antonio.gomez.iglesias@linux.intel.com,
Alyssa Milburn <alyssa.milburn@intel.com>
Subject: Re: [RESEND][PATCH 1/6] x86/bugs: Add asm helpers for executing VERW
Date: Tue, 24 Oct 2023 23:28:18 -0700 [thread overview]
Message-ID: <20231025062818.7kaerqklaut7dg5r@desk> (raw)
In-Reply-To: <f620c7d4-6345-4ad0-8a45-c8089e3c34df@citrix.com>
On Sat, Oct 21, 2023 at 12:55:45AM +0100, Andrew Cooper wrote:
> On 20/10/2023 9:44 pm, Pawan Gupta wrote:
> > +#define EXEC_VERW \
> > + __EXEC_VERW(551f); \
> > + /* nopl __KERNEL_DS(%rax) */ \
> > + .byte 0x0f, 0x1f, 0x80, 0x00, 0x00; \
> > +551: .word __KERNEL_DS; \
>
> Is this actually wise from a perf point of view?
>
> You're causing a data access to the instruction stream, and not only
> that, the immediate next instruction. Some parts don't take kindly to
> snoops hitting L1I.
>
> A better option would be to simply have
>
> .section .text.entry
> .align CACHELINE
> mds_verw_sel:
> .word __KERNEL_DS
> int3
> .align CACHELINE
>
>
> And then just have EXEC_VERW be
>
> verw mds_verw_sel(%rip)
>
> in the fastpaths. That keeps the memory operand in .text.entry it works
> on Meltdown-vulnerable CPUs, but creates effectively a data cacheline
> that isn't mixed into anywhere in the frontend, which also gets far
> better locality of reference.
With .text.entry section I am getting getting below warnings and an
error:
-----------------------------------------------------------------
LD vmlinux.o
vmlinux.o: warning: objtool: .text.entry+0x0: unreachable instruction
vmlinux.o: warning: objtool: .text.entry+0x40: unreachable instruction
vmlinux.o: warning: objtool: .text.entry+0x80: unreachable instruction
vmlinux.o: warning: objtool: .text.entry+0xc0: unreachable instruction
vmlinux.o: warning: objtool: .text.entry+0x100: unreachable instruction
vmlinux.o: warning: objtool: .text.entry+0x140: unreachable instruction
vmlinux.o: warning: objtool: .text.entry+0x180: unreachable instruction
vmlinux.o: warning: objtool: .text.entry+0x1c0: unreachable instruction
vmlinux.o: warning: objtool: .text.entry+0x200: unreachable instruction
vmlinux.o: warning: objtool: .text.entry+0x240: unreachable instruction
vmlinux.o: warning: objtool: .text.entry+0x280: unreachable instruction
vmlinux.o: warning: objtool: .text.entry+0x2c0: unreachable instruction
vmlinux.o: warning: objtool: .text.entry+0x300: unreachable instruction
vmlinux.o: warning: objtool: .altinstr_replacement+0x2c: relocation to !ENDBR: .text.entry+0x0
vmlinux.o: warning: objtool: .altinstr_replacement+0x1c4: relocation to !ENDBR: .text.entry+0x0
vmlinux.o: warning: objtool: .altinstr_replacement+0x1d0: relocation to !ENDBR: .text.entry+0x0
vmlinux.o: warning: objtool: .altinstr_replacement+0x2d2: relocation to !ENDBR: .text.entry+0x80
vmlinux.o: warning: objtool: .altinstr_replacement+0x5d5: relocation to !ENDBR: .text.entry+0xc0
OBJCOPY modules.builtin.modinfo
GEN modules.builtin
MODPOST vmlinux.symvers
UPD include/generated/utsversion.h
CC init/version-timestamp.o
LD .tmp_vmlinux.kallsyms1
ld: error: unplaced orphan section `.text.entry' from `vmlinux.o'
make[2]: *** [scripts/Makefile.vmlinux:36: vmlinux] Error 1
-----------------------------------------------------------------
... because my config has CONFIG_LD_ORPHAN_WARN_LEVEL="error" and
objtool needs to be told about this entry.
Do you think its worth fighting these warnings and error, or simply use
.rodata section for verw memory operand?
next prev parent reply other threads:[~2023-10-25 6:28 UTC|newest]
Thread overview: 42+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-10-20 20:44 [PATCH 0/6] Delay VERW Pawan Gupta
2023-10-20 20:44 ` [PATCH 1/6] x86/bugs: Add asm helpers for executing VERW Pawan Gupta
2023-10-20 23:13 ` Sean Christopherson
2023-10-21 1:00 ` Pawan Gupta
2023-10-20 23:55 ` [RESEND][PATCH " Andrew Cooper
2023-10-21 1:18 ` Pawan Gupta
2023-10-21 1:33 ` Andrew Cooper
2023-10-21 2:21 ` Pawan Gupta
2023-10-23 18:08 ` Josh Poimboeuf
2023-10-23 19:09 ` Pawan Gupta
2023-10-25 6:28 ` Pawan Gupta [this message]
2023-10-25 7:22 ` Peter Zijlstra
2023-10-25 7:52 ` Andrew Cooper
2023-10-25 8:02 ` Peter Zijlstra
2023-10-25 15:27 ` Pawan Gupta
[not found] ` <6439a094-23a6-4de3-aa41-bd033163e044@citrix.com>
2023-10-22 16:16 ` [PATCH " Peter Zijlstra
2023-10-20 20:45 ` [PATCH 2/6] x86/entry_64: Add VERW just before userspace transition Pawan Gupta
2023-10-23 18:22 ` Josh Poimboeuf
2023-10-23 19:13 ` Pawan Gupta
2023-10-23 19:17 ` Dave Hansen
2023-10-23 18:35 ` Josh Poimboeuf
2023-10-23 21:04 ` Pawan Gupta
2023-10-23 21:47 ` Josh Poimboeuf
2023-10-23 22:30 ` Pawan Gupta
2023-10-23 22:45 ` Dave Hansen
2023-10-24 0:00 ` Pawan Gupta
2023-10-20 20:45 ` [PATCH 3/6] x86/entry_32: " Pawan Gupta
2023-10-20 23:49 ` Andi Kleen
2023-10-21 1:28 ` Pawan Gupta
2023-10-20 20:45 ` [PATCH 4/6] x86/bugs: Use ALTERNATIVE() instead of mds_user_clear static key Pawan Gupta
2023-10-23 18:48 ` Josh Poimboeuf
2023-10-23 21:09 ` Pawan Gupta
2023-10-20 20:45 ` [PATCH 5/6] x86/bugs: Cleanup mds_user_clear Pawan Gupta
2023-10-23 8:51 ` Nikolay Borisov
2023-10-23 16:06 ` Pawan Gupta
2023-10-20 20:45 ` [PATCH 6/6] KVM: VMX: Move VERW closer to VMentry for MDS mitigation Pawan Gupta
2023-10-20 22:55 ` Sean Christopherson
2023-10-21 0:46 ` Pawan Gupta
2023-10-23 14:58 ` Sean Christopherson
2023-10-23 17:05 ` Pawan Gupta
2023-10-23 18:56 ` Josh Poimboeuf
2023-10-23 21:17 ` Pawan Gupta
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