linux-kernel - Re: [PATCH v4 5/6] x86/tdx: Move MMIO helpers to common library

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Message-ID: <raojr7elewc6npsph7j5kibeyznsb6ulfid2ksz7i52nldrnu6@uxmdfqoppyef>
Date: Thu, 22 Aug 2024 11:23:52 +0300
From: "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
To: Alexey Gladkov <legion@...nel.org>
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 5/6] x86/tdx: Move MMIO helpers to common library

On Wed, Aug 21, 2024 at 04:24:37PM +0200, Alexey Gladkov wrote:
> @@ -124,3 +125,127 @@ void memset_io(volatile void __iomem *a, int b, size_t c)
>  	}
>  }
>  EXPORT_SYMBOL(memset_io);
> +
> +int __get_iomem(char *src, char *buf, size_t size)
> +{
> +	/*

This comment (and comment for __put_iomem()) has to be updated to be less
SEV-centric.

> +	 * This function uses __get_user() independent of whether kernel or user
> +	 * memory is accessed. This works fine because __get_user() does no
> +	 * sanity checks of the pointer being accessed. All that it does is
> +	 * to report when the access failed.
> +	 *
> +	 * Also, this function runs in atomic context, so __get_user() is not

It is not going to be atomic context for TDX case.

> +	 * allowed to sleep. The page-fault handler detects that it is running
> +	 * in atomic context and will not try to take mmap_sem and handle the
> +	 * fault, so additional pagefault_enable()/disable() calls are not
> +	 * needed.
> +	 *
> +	 * The access can't be done via copy_from_user() here because
> +	 * mmio_read_mem() must not use string instructions to access unsafe
> +	 * memory. The reason is that MOVS is emulated by the #VC handler by
> +	 * splitting the move up into a read and a write and taking a nested #VC
> +	 * exception on whatever of them is the MMIO access. Using string
> +	 * instructions here would cause infinite nesting.

#VC is SEV specific.

> +	 */

-- 
  Kiryl Shutsemau / Kirill A. Shutemov