linux-kernel - Re: [PATCH v3 2/7] x86/sgx: Add infrastructure to identify SGX EPC pages

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Date:   Fri, 30 Jul 2021 13:35:42 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     "Luck, Tony" <tony.luck@...el.com>
Cc:     Sean Christopherson <seanjc@...gle.com>,
        Jarkko Sakkinen <jarkko@...nel.org>, x86@...nel.org,
        linux-kernel@...r.kernel.org, Matthew Wilcox <willy@...radead.org>
Subject: Re: [PATCH v3 2/7] x86/sgx: Add infrastructure to identify SGX EPC
 pages

On 7/30/21 11:44 AM, Luck, Tony wrote:
> @@ -649,6 +650,9 @@ static bool __init sgx_setup_epc_section(u64 phys_addr, u64 size,
>  	}
>  
>  	section->phys_addr = phys_addr;
> +	section->end_phys_addr = phys_addr + size - 1;
> +	xa_store_range(&epc_page_ranges, section->phys_addr,
> +		       section->end_phys_addr, section, GFP_KERNEL);

That is compact, but how much memory does it eat?  I'm a little worried
about this hunk of xa_store_range():

>                 do {
>                         xas_set_range(&xas, first, last);
>                         xas_store(&xas, entry);
>                         if (xas_error(&xas))
>                                 goto unlock;
>                         first += xas_size(&xas);
>                 } while (first <= last);

That makes it look like it's iterating over the whole range and making
loads of individual array instead of doing something super clever like
keeping an extent-style structure.

Let's say we have 1TB of EPC.  How big is the array to store these
indexes?  Would this be more compact if instead of doing a physical
address range:

	xa_store_range(&epc_page_ranges,
		       section->phys_addr,
		       section->end_phys_addr, ...);

... you did it based on PFNs:

	xa_store_range(&epc_page_ranges,
		       section->phys_addr     >> PAGE_SHIFT,
		       section->end_phys_addr >> PAGE_SHIFT, ...);

SGX sections are at *least* page-aligned, so this should be fine.