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

[Sathyanarayanan Kuppuswamy]

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@xxxxxxxxxxxxxxx/
> > >
> > > 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@xxxxxxxxxxxxxxx/T/#maf7e5f6894548972c5de71f607199a79645856ff

--
Sathyanarayanan Kuppuswamy
Linux Kernel Developer