linux-kernel - Re: [PATCH v2] x86: Stop relying on magic jmp behavior for early_idt

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Message-ID: <555F98B4.3030308@zytor.com>
Date:	Fri, 22 May 2015 13:59:32 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Andy Lutomirski <luto@...capital.net>,
	Ingo Molnar <mingo@...nel.org>
CC:	Andy Lutomirski <luto@...nel.org>, X86 ML <x86@...nel.org>,
	"H.J. Lu" <hjl.tools@...il.com>, Borislav Petkov <bp@...en8.de>,
	Jan Beulich <JBeulich@...e.com>,
	Binutils <binutils@...rceware.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2] x86: Stop relying on magic jmp behavior for early_idt_handlers

On 05/21/2015 11:51 PM, Andy Lutomirski wrote:
>>>
>>>  #ifdef __KERNEL__
>>> +
>>> +/*
>>> + * early_idt_handlers is an array of entry points.  For simplicity, it's
>>> + * a real array.  We allocate nine bytes for each entry: two one-byte
>>> + * push instructions and a five-byte jump in the worst case.
>>> + */
>>> +#define EARLY_IDT_HANDLER_STRIDE 9
>>
>> So how come that two plus five equals nine? ;-)
> 
> A "one-byte push" would be pretty impressive :)
> 

Well, non-REX register pushes are just one byte.  However, these take an
immediate constant.

>>
>> Ditto for the 64-bit side.
> 
> Sounds good.  I'll do all this tomorrow and resend.
> 

Thank you!  Since I trust you, I'll send you this in advance:

Acked-by: H. Peter Anvin <hpa@...ux.intel.com>


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