linux-kernel - Re: [PATCH v3 01/21] x86/virt/tdx: Detect SEAM

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Date:   Tue, 26 Apr 2022 16:28:00 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     Kai Huang <kai.huang@...el.com>, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org
Cc:     seanjc@...gle.com, pbonzini@...hat.com, len.brown@...el.com,
        tony.luck@...el.com, rafael.j.wysocki@...el.com,
        reinette.chatre@...el.com, dan.j.williams@...el.com,
        peterz@...radead.org, ak@...ux.intel.com,
        kirill.shutemov@...ux.intel.com,
        sathyanarayanan.kuppuswamy@...ux.intel.com,
        isaku.yamahata@...el.com
Subject: Re: [PATCH v3 01/21] x86/virt/tdx: Detect SEAM

On 4/26/22 16:12, Kai Huang wrote:
> Hi Dave,
> 
> Thanks for review!
> 
> On Tue, 2022-04-26 at 13:21 -0700, Dave Hansen wrote:
>>> +config INTEL_TDX_HOST
>>> +	bool "Intel Trust Domain Extensions (TDX) host support"
>>> +	default n
>>> +	depends on CPU_SUP_INTEL
>>> +	depends on X86_64
>>> +	help
>>> +	  Intel Trust Domain Extensions (TDX) protects guest VMs from
>>> malicious
>>> +	  host and certain physical attacks.  This option enables necessary
>>> TDX
>>> +	  support in host kernel to run protected VMs.
>>> +
>>> +	  If unsure, say N.
>>
>> Nothing about KVM?
> 
> I'll add KVM into the context. How about below?
> 
> "Intel Trust Domain Extensions (TDX) protects guest VMs from malicious
> host and certain physical attacks.  This option enables necessary TDX
> support in host kernel to allow KVM to run protected VMs called Trust
> Domains (TD)."

What about a dependency?  Isn't this dead code without CONFIG_KVM=y/m?

>>> diff --git a/arch/x86/virt/vmx/tdx/tdx.c b/arch/x86/virt/vmx/tdx/tdx.c
>>> new file mode 100644
>>> index 000000000000..03f35c75f439
>>> --- /dev/null
>>> +++ b/arch/x86/virt/vmx/tdx/tdx.c
>>> @@ -0,0 +1,102 @@
>>> +// SPDX-License-Identifier: GPL-2.0
>>> +/*
>>> + * Copyright(c) 2022 Intel Corporation.
>>> + *
>>> + * Intel Trusted Domain Extensions (TDX) support
>>> + */
>>> +
>>> +#define pr_fmt(fmt)	"tdx: " fmt
>>> +
>>> +#include <linux/types.h>
>>> +#include <linux/cpumask.h>
>>> +#include <asm/msr-index.h>
>>> +#include <asm/msr.h>
>>> +#include <asm/cpufeature.h>
>>> +#include <asm/cpufeatures.h>
>>> +#include <asm/tdx.h>
>>> +
>>> +/* Support Intel Secure Arbitration Mode Range Registers (SEAMRR) */
>>> +#define MTRR_CAP_SEAMRR			BIT(15)
>>> +
>>> +/* Core-scope Intel SEAMRR base and mask registers. */
>>> +#define MSR_IA32_SEAMRR_PHYS_BASE	0x00001400
>>> +#define MSR_IA32_SEAMRR_PHYS_MASK	0x00001401
>>> +
>>> +#define SEAMRR_PHYS_BASE_CONFIGURED	BIT_ULL(3)
>>> +#define SEAMRR_PHYS_MASK_ENABLED	BIT_ULL(11)
>>> +#define SEAMRR_PHYS_MASK_LOCKED		BIT_ULL(10)
>>> +
>>> +#define SEAMRR_ENABLED_BITS	\
>>> +	(SEAMRR_PHYS_MASK_ENABLED | SEAMRR_PHYS_MASK_LOCKED)
>>> +
>>> +/* BIOS must configure SEAMRR registers for all cores consistently */
>>> +static u64 seamrr_base, seamrr_mask;
>>> +
>>> +static bool __seamrr_enabled(void)
>>> +{
>>> +	return (seamrr_mask & SEAMRR_ENABLED_BITS) == SEAMRR_ENABLED_BITS;
>>> +}
>>
>> But there's no case where seamrr_mask is non-zero and where
>> _seamrr_enabled().  Why bother checking the SEAMRR_ENABLED_BITS?
> 
> seamrr_mask will only be non-zero when SEAMRR is enabled by BIOS, otherwise it
> is 0.  It will also be cleared when BIOS mis-configuration is detected on any
> AP.  SEAMRR_ENABLED_BITS is used to check whether SEAMRR is enabled.

The point is that this could be:

	return !!seamrr_mask;


>>> +static void detect_seam_ap(struct cpuinfo_x86 *c)
>>> +{
>>> +	u64 base, mask;
>>> +
>>> +	/*
>>> +	 * Don't bother to detect this AP if SEAMRR is not
>>> +	 * enabled after earlier detections.
>>> +	 */
>>> +	if (!__seamrr_enabled())
>>> +		return;
>>> +
>>> +	rdmsrl(MSR_IA32_SEAMRR_PHYS_BASE, base);
>>> +	rdmsrl(MSR_IA32_SEAMRR_PHYS_MASK, mask);
>>> +
>>
>> This is the place for a comment about why the values have to be equal.
> 
> I'll add below:
> 
> /* BIOS must configure SEAMRR consistently across all cores */

What happens if the BIOS doesn't do this?  What actually breaks?  In
other words, do we *NEED* error checking here?

>>> +	if (base == seamrr_base && mask == seamrr_mask)
>>> +		return;
>>> +
>>> +	pr_err("Inconsistent SEAMRR configuration by BIOS\n");
>>> +	/* Mark SEAMRR as disabled. */
>>> +	seamrr_base = 0;
>>> +	seamrr_mask = 0;
>>> +}
>>> +
>>> +static void detect_seam(struct cpuinfo_x86 *c)
>>> +{
>>> +	if (c == &boot_cpu_data)
>>> +		detect_seam_bsp(c);
>>> +	else
>>> +		detect_seam_ap(c);
>>> +}
>>> +
>>> +void tdx_detect_cpu(struct cpuinfo_x86 *c)
>>> +{
>>> +	detect_seam(c);
>>> +}
>>
>> The extra function looks a bit silly here now.  Maybe this gets filled
>> out later, but it's goofy-looking here.
> 
> Thomas suggested to put all TDX detection related in one function call, so I
> added tdx_detect_cpu().  I'll move this to the next patch when detecting TDX
> KeyIDs.

That's fine, or just add a comment or a changelog sentence about this
being filled out later.