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Date: Fri, 11 Dec 2015 12:09:10 -0800
From: Andy Lutomirski <luto@...capital.net>
To: Tony Luck <tony.luck@...el.com>
Cc: Ingo Molnar <mingo@...nel.org>, Borislav Petkov <bp@...en8.de>,
Andrew Morton <akpm@...ux-foundation.org>,
Andy Lutomirski <luto@...nel.org>,
Dan Williams <dan.j.williams@...el.com>,
"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
"linux-mm@...ck.org" <linux-mm@...ck.org>,
linux-nvdimm <linux-nvdimm@...1.01.org>, X86 ML <x86@...nel.org>
Subject: Re: [PATCHV2 3/3] x86, ras: Add mcsafe_memcpy() function to recover
from machine checks
On Thu, Dec 10, 2015 at 4:21 PM, Tony Luck <tony.luck@...el.com> wrote:
> Using __copy_user_nocache() as inspiration create a memory copy
> routine for use by kernel code with annotations to allow for
> recovery from machine checks.
>
> Notes:
> 1) Unlike the original we make no attempt to copy all the bytes
> up to the faulting address. The original achieves that by
> re-executing the failing part as a byte-by-byte copy,
> which will take another page fault. We don't want to have
> a second machine check!
> 2) Likewise the return value for the original indicates exactly
> how many bytes were not copied. Instead we provide the physical
> address of the fault (thanks to help from do_machine_check()
> 3) Provide helpful macros to decode the return value.
>
> Signed-off-by: Tony Luck <tony.luck@...el.com>
> ---
> arch/x86/include/asm/uaccess_64.h | 5 +++
> arch/x86/kernel/x8664_ksyms_64.c | 2 +
> arch/x86/lib/copy_user_64.S | 91 +++++++++++++++++++++++++++++++++++++++
> 3 files changed, 98 insertions(+)
>
> diff --git a/arch/x86/include/asm/uaccess_64.h b/arch/x86/include/asm/uaccess_64.h
> index f2f9b39b274a..779cb0e77ecc 100644
> --- a/arch/x86/include/asm/uaccess_64.h
> +++ b/arch/x86/include/asm/uaccess_64.h
> @@ -216,6 +216,11 @@ __copy_to_user_inatomic(void __user *dst, const void *src, unsigned size)
> extern long __copy_user_nocache(void *dst, const void __user *src,
> unsigned size, int zerorest);
>
> +extern u64 mcsafe_memcpy(void *dst, const void __user *src,
> + unsigned size);
> +#define COPY_HAD_MCHECK(ret) ((ret) & BIT(63))
> +#define COPY_MCHECK_PADDR(ret) ((ret) & ~BIT(63))
> +
> static inline int
> __copy_from_user_nocache(void *dst, const void __user *src, unsigned size)
> {
> diff --git a/arch/x86/kernel/x8664_ksyms_64.c b/arch/x86/kernel/x8664_ksyms_64.c
> index a0695be19864..ec988c92c055 100644
> --- a/arch/x86/kernel/x8664_ksyms_64.c
> +++ b/arch/x86/kernel/x8664_ksyms_64.c
> @@ -37,6 +37,8 @@ EXPORT_SYMBOL(__copy_user_nocache);
> EXPORT_SYMBOL(_copy_from_user);
> EXPORT_SYMBOL(_copy_to_user);
>
> +EXPORT_SYMBOL(mcsafe_memcpy);
> +
> EXPORT_SYMBOL(copy_page);
> EXPORT_SYMBOL(clear_page);
>
> diff --git a/arch/x86/lib/copy_user_64.S b/arch/x86/lib/copy_user_64.S
> index 982ce34f4a9b..ffce93cbc9a5 100644
> --- a/arch/x86/lib/copy_user_64.S
> +++ b/arch/x86/lib/copy_user_64.S
> @@ -319,3 +319,94 @@ ENTRY(__copy_user_nocache)
> _ASM_EXTABLE(21b,50b)
> _ASM_EXTABLE(22b,50b)
> ENDPROC(__copy_user_nocache)
> +
> +/*
> + * mcsafe_memcpy - Uncached memory copy with machine check exception handling
> + * Note that we only catch machine checks when reading the source addresses.
> + * Writes to target are posted and don't generate machine checks.
> + * This will force destination/source out of cache for more performance.
> + */
> +ENTRY(mcsafe_memcpy)
> + cmpl $8,%edx
> + jb 20f /* less then 8 bytes, go to byte copy loop */
> +
> + /* check for bad alignment of destination */
> + movl %edi,%ecx
> + andl $7,%ecx
> + jz 102f /* already aligned */
> + subl $8,%ecx
> + negl %ecx
> + subl %ecx,%edx
> +0: movb (%rsi),%al
> + movb %al,(%rdi)
> + incq %rsi
> + incq %rdi
> + decl %ecx
> + jnz 100b
> +102:
> + movl %edx,%ecx
> + andl $63,%edx
> + shrl $6,%ecx
> + jz 17f
> +1: movq (%rsi),%r8
> +2: movq 1*8(%rsi),%r9
> +3: movq 2*8(%rsi),%r10
> +4: movq 3*8(%rsi),%r11
> + movnti %r8,(%rdi)
> + movnti %r9,1*8(%rdi)
> + movnti %r10,2*8(%rdi)
> + movnti %r11,3*8(%rdi)
> +9: movq 4*8(%rsi),%r8
> +10: movq 5*8(%rsi),%r9
> +11: movq 6*8(%rsi),%r10
> +12: movq 7*8(%rsi),%r11
> + movnti %r8,4*8(%rdi)
> + movnti %r9,5*8(%rdi)
> + movnti %r10,6*8(%rdi)
> + movnti %r11,7*8(%rdi)
> + leaq 64(%rsi),%rsi
> + leaq 64(%rdi),%rdi
> + decl %ecx
> + jnz 1b
> +17: movl %edx,%ecx
> + andl $7,%edx
> + shrl $3,%ecx
> + jz 20f
> +18: movq (%rsi),%r8
> + movnti %r8,(%rdi)
> + leaq 8(%rsi),%rsi
> + leaq 8(%rdi),%rdi
> + decl %ecx
> + jnz 18b
> +20: andl %edx,%edx
> + jz 23f
> + movl %edx,%ecx
> +21: movb (%rsi),%al
> + movb %al,(%rdi)
> + incq %rsi
> + incq %rdi
> + decl %ecx
> + jnz 21b
> +23: xorl %eax,%eax
> + sfence
> + ret
> +
> + .section .fixup,"ax"
> +30:
> + sfence
> + /* do_machine_check() sets %eax return value */
> + ret
> + .previous
> +
> + _ASM_MCEXTABLE(0b,30b)
> + _ASM_MCEXTABLE(1b,30b)
> + _ASM_MCEXTABLE(2b,30b)
> + _ASM_MCEXTABLE(3b,30b)
> + _ASM_MCEXTABLE(4b,30b)
> + _ASM_MCEXTABLE(9b,30b)
> + _ASM_MCEXTABLE(10b,30b)
> + _ASM_MCEXTABLE(11b,30b)
> + _ASM_MCEXTABLE(12b,30b)
> + _ASM_MCEXTABLE(18b,30b)
> + _ASM_MCEXTABLE(21b,30b)
> +ENDPROC(mcsafe_memcpy)
I still don't get the BIT(63) thing. Can you explain it?
--Andy
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