linux-kernel - Re: [PATCH v5 6/6] x86/tdx: Implement MOVS for MMIO

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Message-ID: <ZtDAuDAyGevAhSj-@example.org>
Date: Thu, 29 Aug 2024 20:40:56 +0200
From: Alexey Gladkov <legion@...nel.org>
To: "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
Cc: Tom Lendacky <thomas.lendacky@....com>, 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 v5 6/6] x86/tdx: Implement MOVS for MMIO

On Thu, Aug 29, 2024 at 03:44:55PM +0300, Kirill A. Shutemov wrote:
> On Wed, Aug 28, 2024 at 12:44:36PM +0200, Alexey Gladkov wrote:
> > From: "Alexey Gladkov (Intel)" <legion@...nel.org>
> > 
> > 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.
> > 
> > This implementation is based on the same principle as in SEV. It splits
> > MOVS into separate read and write operations, which in turn can cause
> > nested #VEs depending on which of the arguments caused the first #VE.
> > 
> > The difference with the SEV implementation is the execution context. SEV
> > code is executed in atomic context. Exception handler in TDX is executed
> > with interrupts enabled. That's why the approach to locking is
> > different. In TDX, mmap_lock is taken to verify and emulate the
> > instruction.
> > 
> > Another difference is how the read and write instructions are executed
> > for MOVS emulation. While in SEV each read/write operation returns to
> > user space, in TDX these operations are performed from the kernel
> > context.
> 
> It looks like SEV only returns to userspace to retry the instruction after
> stepping on failed __get_user()/__put_user(), unrolling back to
> vc_raw_handle_exception() and handling page fault there.

In vc_handle_mmio_movs() if regs->cx is not zero we return ES_RETRY. The
vc_handle_mmio(), vc_handle_exitcode() return it as is. In
vc_raw_handle_exception() if vc_handle_exitcode() returns ES_RETRY then we
just return true. So, the ES_RETRY is not further visible.

Or am I missing something?

> 
> But I'm not sure what happens with #VC inside vc_read_mem() and
> vc_write_mem(). Can the #VC exception be nested? Tom?
> 
> -- 
>   Kiryl Shutsemau / Kirill A. Shutemov
> 

-- 
Rgrds, legion