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

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Message-ID: <6n6tn2t4tabiclau7cpav6b7kisuotqwg246wp5lxgfngoyyl2@674snretgst5>
Date: Thu, 29 Aug 2024 15:44:55 +0300
From: "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
To: Alexey Gladkov <legion@...nel.org>, 
	Tom Lendacky <thomas.lendacky@....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 v5 6/6] x86/tdx: Implement MOVS for MMIO

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.

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