linux-kernel - Re: [RFC PATCH] x86/insn: support decode MOVSXD instruction for MMIO

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Message-ID: <e9c3019f-936f-8bcc-d20f-16220993ff1c@amd.com>
Date:   Fri, 31 Mar 2023 09:33:31 -0500
From:   Tom Lendacky <thomas.lendacky@....com>
To:     "Kirill A. Shutemov" <kirill@...temov.name>
Cc:     David Laight <David.Laight@...LAB.COM>,
        'Wu Zongyong' <wuzongyong@...ux.alibaba.com>,
        "kirill.shutemov@...ux.intel.com" <kirill.shutemov@...ux.intel.com>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "dave.hansen@...ux.intel.com" <dave.hansen@...ux.intel.com>,
        "x86@...nel.org" <x86@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "tony.luck@...el.com" <tony.luck@...el.com>,
        "wutu.xq2@...ux.alibaba.com" <wutu.xq2@...ux.alibaba.com>
Subject: Re: [RFC PATCH] x86/insn: support decode MOVSXD instruction for MMIO



On 3/31/23 09:09, Kirill A. Shutemov wrote:
> On Fri, Mar 31, 2023 at 08:40:30AM -0500, Tom Lendacky wrote:
>> On 3/31/23 05:06, Kirill A. Shutemov wrote:
>>> On Fri, Mar 31, 2023 at 08:49:48AM +0000, David Laight wrote:
>>>> From: Wu Zongyong
>>>>> Sent: 31 March 2023 03:24
>>>>>
>>>>> On Thu, Mar 30, 2023 at 03:39:51PM +0300, kirill.shutemov@...ux.intel.com wrote:
>>>>>> On Wed, Mar 29, 2023 at 10:59:37AM +0800, Wu Zongyong wrote:
>>>>>>> It seems MOVSXD which opcode is 0x63 is not handled, support
>>>>>>> to decode it in insn_decode_mmio().
>>>>>>
>>>>>> Do you have a particular user in mind?
>>>>> To be honest, I don't find a specific user which uses the MOVSXD.
>>>>>
>>>>> But both Intel and AMD's instructions reference contains MOVSXD and lots
>>>>> of MOVSXD instructions occur when I "objdump -S vmlinux", so I think it
>>>>> may be useful to support it in insn_decode_mmio().
>>>>>
>>>>> Are there some special consideration about this instruction?
>>>>
>>>> It is a sign-extending memory read (32bit to 64bit).
>>>> You pretty much never want to do that to a device register.
>>>> Also kernel code should be using readl() (etc) which do
>>>> unsigned reads.
>>>> So they should never happen for mmio.
>>>>
>>>> Of course, if you mmap() PCIe space directly into a program's
>>>> address space anything might happen ...
>>>
>>> There are two users of the interface: TDX and SEV. TDX doesn't allow
>>> userspace MMIO. SEV *seems* allows it, but I am not sure how it is safe.
>>>
>>> Tom?
>>
>> The insn_decode_mmio() function is only called by the SEV/TDX related code
>> and is specifically MMIO oriented. As David said, this instruction is likely
>> not being used for that in the kernel. If we come across a case where this
>> is used, we can look at how it is being used in that situation and it can be
>> addressed then.
> 
> I was asking if SEV supports userspace MMIO. And if yes, how do you make
> it safe?
> 

No, SEV doesn't support userspace MMIO.

Thanks,
Tom