linux-kernel - Re: [PATCH v4 6/6] x86/tdx: Implement movs for MMIO

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Message-ID: <ZsoQ8GCWTHgY3H_e@example.org>
Date: Sat, 24 Aug 2024 18:57:20 +0200
From: Alexey Gladkov <legion@...nel.org>
To: "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
Cc: linux-kernel@...r.kernel.org, linux-coco@...ts.linux.dev,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Yuan Yao <yuan.yao@...el.com>,
	Geert Uytterhoeven <geert@...ux-m68k.org>,
	Yuntao Wang <ytcoode@...il.com>, Kai Huang <kai.huang@...el.com>,
	Baoquan He <bhe@...hat.com>, Oleg Nesterov <oleg@...hat.com>,
	cho@...rosoft.com, decui@...rosoft.com, John.Starks@...rosoft.com
Subject: Re: [PATCH v4 6/6] x86/tdx: Implement movs for MMIO

On Thu, Aug 22, 2024 at 11:28:14AM +0300, Kirill A. Shutemov wrote:
> On Wed, Aug 21, 2024 at 04:24:38PM +0200, Alexey Gladkov wrote:
> > From: "Alexey Gladkov (Intel)" <legion@...nel.org>
> > 
> 
> Please capitalize MOVS in the subject.
> 
> > Add emulation of the MOVS instruction on MMIO regions. MOVS emulation
> > consists of dividing it into a series of read and write operations,
> > which in turn will be validated separately.
> 
> Commit message is pretty sparse. I think we need to elaborate on the
> similarities and differences with SEV implementation. Locking context
> difference is important.

Agree.

> > diff --git a/arch/x86/include/asm/processor.h b/arch/x86/include/asm/processor.h
> > index a75a07f4931f..45136b1b02cc 100644
> > --- a/arch/x86/include/asm/processor.h
> > +++ b/arch/x86/include/asm/processor.h
> > @@ -503,6 +503,10 @@ struct thread_struct {
> >  	struct thread_shstk	shstk;
> >  #endif
> >  
> > +#ifdef CONFIG_INTEL_TDX_GUEST
> > +	unsigned long		mmio_emul;
> > +#endif
> > +
> >  	/* Floating point and extended processor state */
> >  	struct fpu		fpu;
> >  	/*
> 
> Hm. Do we need to track exact target address in the thread struct?
> Wouldn't be single bit be enough to allow MMIO to userspace address from a
> kernel regs->ip?

The flag will identify that a nested exception happened, but it will not
be clear which address cause it.

Perhaps you are right and this approach is unnecessarily paranoid. 

> There is space for the flag next to iopl_warn.

Yes, I can use just a flag to identify a nested exception.

-- 
Rgrds, legion