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

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Message-ID: <20230331100620.mkqg72vwhmlliunn@box.shutemov.name>
Date:   Fri, 31 Mar 2023 13:06:20 +0300
From:   "Kirill A. Shutemov" <kirill@...temov.name>
To:     David Laight <David.Laight@...LAB.COM>,
        Tom Lendacky <thomas.lendacky@....com>
Cc:     '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 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?

-- 
  Kiryl Shutsemau / Kirill A. Shutemov