Re: [PATCH Part1 RFC v4 24/36] x86/compressed/acpi: move EFI config table access to common code

From: Michael Roth <michael.roth@amd.com>
To: Borislav Petkov <bp@alien8.de>
Cc: Brijesh Singh <brijesh.singh@amd.com>,
	x86@kernel.org, linux-kernel@vger.kernel.org,
	kvm@vger.kernel.org, linux-efi@vger.kernel.org,
	platform-driver-x86@vger.kernel.org, linux-coco@lists.linux.dev,
	linux-mm@kvack.org, linux-crypto@vger.kernel.org,
	Thomas Gleixner <tglx@linutronix.de>,
	Ingo Molnar <mingo@redhat.com>, Joerg Roedel <jroedel@suse.de>,
	Tom Lendacky <thomas.lendacky@amd.com>,
	"H. Peter Anvin" <hpa@zytor.com>,
	Ard Biesheuvel <ardb@kernel.org>,
	Paolo Bonzini <pbonzini@redhat.com>,
	Sean Christopherson <seanjc@google.com>,
	Vitaly Kuznetsov <vkuznets@redhat.com>,
	Wanpeng Li <wanpengli@tencent.com>,
	Jim Mattson <jmattson@google.com>,
	Andy Lutomirski <luto@kernel.org>,
	Dave Hansen <dave.hansen@linux.intel.com>,
	Sergio Lopez <slp@redhat.com>, Peter Gonda <pgonda@google.com>,
	Peter Zijlstra <peterz@infradead.org>,
	Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>,
	David Rientjes <rientjes@google.com>,
	Dov Murik <dovmurik@linux.ibm.com>,
	Tobin Feldman-Fitzthum <tobin@ibm.com>,
	Vlastimil Babka <vbabka@suse.cz>,
	tony.luck@intel.com, brijesh.ksingh@gmail.com
Subject: Re: [PATCH Part1 RFC v4 24/36] x86/compressed/acpi: move EFI config table access to common code
Date: Thu, 19 Aug 2021 18:42:58 -0500	[thread overview]
Message-ID: <20210819234258.drlyzowk7y3t5wnw@amd.com> (raw)
In-Reply-To: <YR6QVh3qZUxqsyI+@zn.tnic>

On Thu, Aug 19, 2021 at 07:09:42PM +0200, Borislav Petkov wrote:
> On Thu, Aug 19, 2021 at 09:58:31AM -0500, Michael Roth wrote:
> > Not sure what you mean here. All the interfaces introduced here are used
> > by acpi.c. There is another helper added later (efi_bp_find_vendor_table())
> > in "enable SEV-SNP-validated CPUID in #VC handler", since it's not used
> > here by acpi.c.
> 
> Maybe I got confused by the amount of changes in a single patch. I'll
> try harder with your v5. :)
> 
> > There is the aforementioned efi_bp_find_vendor_table() that does the
> > simple iteration, but I wasn't sure how to build the "find one of these,
> > but this one is preferred" logic into it in a reasonable way.
> 
> Instead of efi_foreach_conf_entry() you simply do a bog-down simple
> loop and each time you stop at a table, you examine it and overwrite
> pointers, if you've found something better.
> 
> With "overwrite pointers" I mean you cache the pointers to those conf
> tables you iterate over and dig out so that you don't have to do it a
> second time. That is, *if* you need them a second time. I believe you
> call at least efi_bp_get_conf_table() twice... you get the idea.

Sorry I'm still a little confused on how to determine "something better",
since it's acpi.c that decides which GUID is preferred, whereas
efi_find_vendor_table() is a library function with no outside knowledge
other than its arguments, so to return the preferred pointer it would need
both/multiple GUIDs passed in as arguments, wouldn't it? (or a callback)

Another alternative is something like what
drivers/firmware/efi/efi.c:common_tables does, where interesting table
GUIDs are each associated with a pointer, and all the pointers can then
be initialized with to the corresponding table address with a single pass.
But would need to be careful to re-initialize those pointers when BSS gets
cleared, or declare them in __section(".data"). Is that closer to what
you were thinking?

> 
> > I could just call it once for each of these GUIDs though. I was
> > hesitant to do so since it's less efficient than existing code, but if
> > it's worth it for the simplification then I'm all for it.
> 
> Yeah, this is executed once during boot so I don't think you can make it
> more efficient than a single iteration over the config table blobs.

In v5, I've simplified things to just call efi_find_vendor_table() once
for ACPI_20_TABLE_GUID, then once for ACPI_TABLE_GUID if that's not
available. So definitely doesn't sound like what you are suggesting here,
but does at least simplify code and gets rid of the efi_foreach* stuff. But
happy to rework things if you had something else in mind.

> 
> I hope that makes more sense.
> 
> -- 
> Regards/Gruss,
>     Boris.
> 
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