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Message-ID: <20251025100642.GVaPyhMp4CEmsYW3xy@fat_crate.local>
Date: Sat, 25 Oct 2025 12:06:42 +0200
From: Borislav Petkov <bp@...en8.de>
To: Brendan Jackman <jackmanb@...gle.com>
Cc: Andy Lutomirski <luto@...nel.org>,
Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
"Liam R. Howlett" <Liam.Howlett@...cle.com>,
Suren Baghdasaryan <surenb@...gle.com>,
Michal Hocko <mhocko@...e.com>,
Johannes Weiner <hannes@...xchg.org>, Zi Yan <ziy@...dia.com>,
Axel Rasmussen <axelrasmussen@...gle.com>,
Yuanchu Xie <yuanchu@...gle.com>,
Roman Gushchin <roman.gushchin@...ux.dev>, peterz@...radead.org,
dave.hansen@...ux.intel.com, mingo@...hat.com, tglx@...utronix.de,
akpm@...ux-foundation.org, david@...hat.com, derkling@...gle.com,
junaids@...gle.com, linux-kernel@...r.kernel.org,
linux-mm@...ck.org, reijiw@...gle.com, rientjes@...gle.com,
rppt@...nel.org, vbabka@...e.cz, x86@...nel.org,
Yosry Ahmed <yosry.ahmed@...ux.dev>
Subject: Re: [PATCH 02/21] x86/mm/asi: add X86_FEATURE_ASI and asi=
On Wed, Sep 24, 2025 at 02:59:37PM +0000, Brendan Jackman wrote:
> diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
> index 6c42061ca20e581b5192b66c6f25aba38d4f8ff8..9b8330fc1fe31721af39b08b58b729ced78ba803 100644
> --- a/Documentation/admin-guide/kernel-parameters.txt
> +++ b/Documentation/admin-guide/kernel-parameters.txt
> @@ -5324,6 +5324,14 @@
>
> Not specifying this option is equivalent to pti=auto.
>
> + asi= [X86-64] Control Address Space Isolation (ASI), a
> + technology for mitigating CPU vulnerabilities.
> ASI is
> + not yet ready to provide security guarantees but can be
> + enabled for evaluation.
Yeah, no need for such "temporary" statements in the help text since you're
going to have to touch it again once it becomes a full-fledged feature.
> + on - unconditionally enable
> + off - unconditionally disable
"unconditionally" as opposed to some other setting which is conditional?
> +
> pty.legacy_count=
> [KNL] Number of legacy pty's. Overwrites compiled-in
> default number.
> diff --git a/arch/x86/include/asm/asi.h b/arch/x86/include/asm/asi.h
> index 53acdf22fe33efc6ccedbae52b262a904868459a..32a4c04c4be0f6f425c7cbcff4c58f1827a4b4c4 100644
> --- a/arch/x86/include/asm/asi.h
> +++ b/arch/x86/include/asm/asi.h
> @@ -2,4 +2,14 @@
> #ifndef _ASM_X86_ASI_H
> #define _ASM_X86_ASI_H
>
> +#include <asm/cpufeature.h>
> +
> +void asi_check_boottime_disable(void);
> +
> +/* Helper for generic code. Arch code just uses cpu_feature_enabled(). */
> +static inline bool asi_enabled_static(void)
"static" because? There will be a dynamic one too?
> +{
> + return cpu_feature_enabled(X86_FEATURE_ASI);
> +}
> +
> #endif /* _ASM_X86_ASI_H */
> diff --git a/arch/x86/include/asm/cpufeatures.h b/arch/x86/include/asm/cpufeatures.h
> index 4091a776e37aaed67ca93b0a0cd23cc25dbc33d4..3eee24a4cabf3b2131c34596236d8bc8eec05b3b 100644
> --- a/arch/x86/include/asm/cpufeatures.h
> +++ b/arch/x86/include/asm/cpufeatures.h
> @@ -499,6 +499,7 @@
> #define X86_FEATURE_IBPB_EXIT_TO_USER (21*32+14) /* Use IBPB on exit-to-userspace, see VMSCAPE bug */
> #define X86_FEATURE_ABMC (21*32+15) /* Assignable Bandwidth Monitoring Counters */
> #define X86_FEATURE_MSR_IMM (21*32+16) /* MSR immediate form instructions */
> +#define X86_FEATURE_ASI (21*32+17) /* Kernel Address Space Isolation */
I think we really will need to show this in /proc/cpuinfo as it is a real, big
feature which gets proper kernel glue vs some silly CPUID bit.
IOW,
#define X86_FEATURE_ASI (21*32+17) /* "asi" Kernel Address Space Isolation */
^^^^
Not sure, though, when we should make it an ABI - perhaps once the whole pile
has landed...
> /*
> * BUG word(s)
> diff --git a/arch/x86/mm/Makefile b/arch/x86/mm/Makefile
> index 5b9908f13dcfd092897f3778ee56ea4d45bdb868..5ecbff70964f61a903ac96cec3736a7cec1221fd 100644
> --- a/arch/x86/mm/Makefile
> +++ b/arch/x86/mm/Makefile
> @@ -52,6 +52,7 @@ obj-$(CONFIG_ACPI_NUMA) += srat.o
> obj-$(CONFIG_X86_INTEL_MEMORY_PROTECTION_KEYS) += pkeys.o
> obj-$(CONFIG_RANDOMIZE_MEMORY) += kaslr.o
> obj-$(CONFIG_MITIGATION_PAGE_TABLE_ISOLATION) += pti.o
> +obj-$(CONFIG_MITIGATION_ADDRESS_SPACE_ISOLATION) += asi.o
>
> obj-$(CONFIG_X86_MEM_ENCRYPT) += mem_encrypt.o
> obj-$(CONFIG_AMD_MEM_ENCRYPT) += mem_encrypt_amd.o
> diff --git a/arch/x86/mm/asi.c b/arch/x86/mm/asi.c
> new file mode 100644
> index 0000000000000000000000000000000000000000..8c907f3c84f43f66e412ecbfa99e67390d31a66f
> --- /dev/null
> +++ b/arch/x86/mm/asi.c
> @@ -0,0 +1,28 @@
> +// SPDX-License-Identifier: GPL-2.0
> +#include <linux/asi.h>
> +#include <linux/init.h>
> +#include <linux/string.h>
> +
> +#include <asm/cmdline.h>
> +#include <asm/cpufeature.h>
> +
> +void __init asi_check_boottime_disable(void)
> +{
> + bool enabled = false;
> + char arg[4];
> + int ret;
> +
> + ret = cmdline_find_option(boot_command_line, "asi", arg, sizeof(arg));
> + if (ret == 3 && !strncmp(arg, "off", 3)) {
> + enabled = false;
> + pr_info("ASI explicitly disabled by kernel cmdline.\n");
> + } else if (ret == 2 && !strncmp(arg, "on", 2)) {
> + enabled = true;
> + pr_info("ASI enabled.\n");
I'm not sure about those pr_info()s. When it is disabled, you can clear
X86_FEATURE_ASI so you won't see it in /proc/cpuinfo and then it is disabled.
And the same when it is enabled.
> + } else if (ret) {
> + pr_err("Unknown asi= flag '%s', try 'off' or 'on'\n", arg);
> + }
> +
> + if (enabled)
> + setup_force_cpu_cap(X86_FEATURE_ASI);
> +}
Not an early_param() ?
--
Regards/Gruss,
Boris.
https://people.kernel.org/tglx/notes-about-netiquette
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