Re: [PATCH v5 3/3] x86/tdx: Add Quote generation support

From: Sathyanarayanan Kuppuswamy  <sathyanarayanan.kuppuswamy@linux.intel.com>
To: Kai Huang <kai.huang@intel.com>,
	"Kirill A. Shutemov" <kirill@shutemov.name>
Cc: Thomas Gleixner <tglx@linutronix.de>,
	Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
	Dave Hansen <dave.hansen@linux.intel.com>,
	x86@kernel.org, "H . Peter Anvin" <hpa@zytor.com>,
	"Kirill A . Shutemov" <kirill.shutemov@linux.intel.com>,
	Tony Luck <tony.luck@intel.com>, Andi Kleen <ak@linux.intel.com>,
	Wander Lairson Costa <wander@redhat.com>,
	Isaku Yamahata <isaku.yamahata@gmail.com>,
	marcelo.cerri@canonical.com, tim.gardner@canonical.com,
	khalid.elmously@canonical.com, philip.cox@canonical.com,
	linux-kernel@vger.kernel.org
Subject: Re: [PATCH v5 3/3] x86/tdx: Add Quote generation support
Date: Mon, 2 May 2022 19:39:04 -0700	[thread overview]
Message-ID: <ac299f99-7ef6-7fc2-a9fe-7ed2586b9b09@linux.intel.com> (raw)
In-Reply-To: <4cf22a0799cc13fa6f09ffbf0e5d83ed7eeb32ad.camel@intel.com>

On 5/2/22 7:18 PM, Kai Huang wrote:
> On Tue, 2022-05-03 at 04:27 +0300, Kirill A. Shutemov wrote:
>> On Mon, May 02, 2022 at 02:40:26PM +1200, Kai Huang wrote:
>>>
>>>> +
>>>> +	/* Get order for Quote buffer page allocation */
>>>> +	order = get_order(quote_req.len);
>>>> +
>>>> +	/*
>>>> +	 * Allocate buffer to get TD Quote from the VMM.
>>>> +	 * Size needs to be 4KB aligned (which is already
>>>> +	 * met in page allocation).
>>>> +	 */
>>>> +	tdquote = (void *)__get_free_pages(GFP_KERNEL | __GFP_ZERO, order);
>>>> +	if (!tdquote) {
>>>> +		ret = -ENOMEM;
>>>> +		goto quote_failed;
>>>> +	}
>>>
>>> You can use alloc_pages_exact().
>>>
>>>> +
>>>> +	/*
>>>> +	 * Since this buffer will be shared with the VMM via GetQuote
>>>> +	 * hypercall, decrypt it.
>>>> +	 */
>>>> +	ret = set_memory_decrypted((unsigned long)tdquote, 1UL << order);
>>>> +	if (ret)
>>>> +		goto quote_failed;
>>>
>>>
>>> Again, Dave and Andi already commented you should use vmap() to avoid breaking
>>> up the direct-mapping.  Please use vmap() instead.
>>>
>>> https://lore.kernel.org/all/ce0feeec-a949-35f8-3010-b0d69acbbc2e@linux.intel.com/
>>>
>>> Will review the rest later.
>>
>> I would rather convert it to use DMA API for memory allocation. It will
>> tap into swiotlb buffer that already converted and there's no need to
>> touch direct mapping. Both allocation and freeing such memory is cheaper
>> because of that.
>>
> 
> Does each DMA allocation and free internally do the actual private/shared
> conversion?  Or the swiotlb is converted at the beginning at boot and DMA
> allocation will always get the shared buffer automatically?

DMA allocation will always return shared buffer.

> 
> The problem of using DMA API is it will need to bring additional code to use
> platform device, which isn't necessary.

Yes.

> 
> Using vmap() we can still (almost) avoid private/shared conversion at IOCTL time
> by allocating a default size buffer (which is large enough to cover 99% cases,
> etc) at driver initialization time:

Allocating fixed size buffer pool will work for dma buffer allocation
as well.

So the comparison is between platform driver boilerplate code vs vmap
and shared/unshared code addition. It is arguable which is better. I
think it is about preference.

> 
> https://lore.kernel.org/lkml/20220422233418.1203092-2-sathyanarayanan.kuppuswamy@linux.intel.com/T/#maf7e5f6894548972c5de71f607199a79645856ff
> 
> 

-- 
Sathyanarayanan Kuppuswamy
Linux Kernel Developer