linux-kernel - Re: [PATCH v5 16/16] x86/tdx: Add cmdline option to force use of ioremap_host

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Date:   Fri, 8 Oct 2021 18:45:26 -0700
From:   Randy Dunlap <rdunlap@...radead.org>
To:     Kuppuswamy Sathyanarayanan 
        <sathyanarayanan.kuppuswamy@...ux.intel.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Andy Lutomirski <luto@...nel.org>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Richard Henderson <rth@...ddle.net>,
        Thomas Bogendoerfer <tsbogend@...ha.franken.de>,
        James E J Bottomley <James.Bottomley@...senPartnership.com>,
        Helge Deller <deller@....de>,
        "David S . Miller" <davem@...emloft.net>,
        Arnd Bergmann <arnd@...db.de>,
        Jonathan Corbet <corbet@....net>,
        "Michael S . Tsirkin" <mst@...hat.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        David Hildenbrand <david@...hat.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>
Cc:     Peter H Anvin <hpa@...or.com>, Dave Hansen <dave.hansen@...el.com>,
        Tony Luck <tony.luck@...el.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Andi Kleen <ak@...ux.intel.com>,
        Kirill Shutemov <kirill.shutemov@...ux.intel.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Kuppuswamy Sathyanarayanan <knsathya@...nel.org>,
        x86@...nel.org, linux-kernel@...r.kernel.org,
        linux-pci@...r.kernel.org, linux-alpha@...r.kernel.org,
        linux-mips@...r.kernel.org, linux-parisc@...r.kernel.org,
        sparclinux@...r.kernel.org, linux-arch@...r.kernel.org,
        linux-doc@...r.kernel.org,
        virtualization@...ts.linux-foundation.org
Subject: Re: [PATCH v5 16/16] x86/tdx: Add cmdline option to force use of
 ioremap_host_shared

On 10/8/21 5:37 PM, Kuppuswamy Sathyanarayanan wrote:
> diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
> index 91ba391f9b32..0af19cb1a28c 100644
> --- a/Documentation/admin-guide/kernel-parameters.txt
> +++ b/Documentation/admin-guide/kernel-parameters.txt
> @@ -2076,6 +2076,18 @@
>   			1 - Bypass the IOMMU for DMA.
>   			unset - Use value of CONFIG_IOMMU_DEFAULT_PASSTHROUGH.
>   
> +	ioremap_force_shared= [X86_64, CCG]
> +			Force the kernel to use shared memory mappings which do
> +			not use ioremap_host_shared/pcimap_host_shared to opt-in
> +			to shared mappings with the host. This feature is mainly
> +			used by a confidential guest when enabling new drivers
> +			without proper shared memory related changes. Please note
> +			that this option might also allow other non explicitly
> +			enabled drivers to interact with the host in confidential
> +			guest, which could cause other security risks. This option
> +			will also cause BIOS data structures to be shared with the
> +			host, which might open security holes.

Hi,
This cmdline option text should have a little bit more info. Just as an
example/template:

	acpi_apic_instance=	[ACPI, IOAPIC]
			Format: <int>
			2: use 2nd APIC table, if available
			1,0: use 1st APIC table
			default: 0

So what is expected after the "=" sign?...

thanks.
-- 
~Randy