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Fri, 15 Aug 2025 01:37:49 -0700 (PDT) Message-ID: <5c608ebc-e9c2-47ea-a98f-246f7a3d4c30@suse.com> Date: Fri, 15 Aug 2025 10:37:48 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH 15/22] x86/traps: Introduce opt_fred To: Andrew Cooper Cc: Xen-devel , =?UTF-8?Q?Roger_Pau_Monn=C3=A9?= References: <20250808202314.1045968-1-andrew.cooper3@citrix.com> <20250808202314.1045968-16-andrew.cooper3@citrix.com> <0a6b6fce-2cc8-4933-8703-399e4a4fdad6@suse.com> <8adbd032-9da2-481d-8d98-30c9a605ec46@citrix.com> Content-Language: en-US From: Jan Beulich Autocrypt: addr=jbeulich@suse.com; keydata= xsDiBFk3nEQRBADAEaSw6zC/EJkiwGPXbWtPxl2xCdSoeepS07jW8UgcHNurfHvUzogEq5xk hu507c3BarVjyWCJOylMNR98Yd8VqD9UfmX0Hb8/BrA+Hl6/DB/eqGptrf4BSRwcZQM32aZK 7Pj2XbGWIUrZrd70x1eAP9QE3P79Y2oLrsCgbZJfEwCgvz9JjGmQqQkRiTVzlZVCJYcyGGsD /0tbFCzD2h20ahe8rC1gbb3K3qk+LpBtvjBu1RY9drYk0NymiGbJWZgab6t1jM7sk2vuf0Py O9Hf9XBmK0uE9IgMaiCpc32XV9oASz6UJebwkX+zF2jG5I1BfnO9g7KlotcA/v5ClMjgo6Gl MDY4HxoSRu3i1cqqSDtVlt+AOVBJBACrZcnHAUSuCXBPy0jOlBhxPqRWv6ND4c9PH1xjQ3NP nxJuMBS8rnNg22uyfAgmBKNLpLgAGVRMZGaGoJObGf72s6TeIqKJo/LtggAS9qAUiuKVnygo 3wjfkS9A3DRO+SpU7JqWdsveeIQyeyEJ/8PTowmSQLakF+3fote9ybzd880fSmFuIEJldWxp Y2ggPGpiZXVsaWNoQHN1c2UuY29tPsJgBBMRAgAgBQJZN5xEAhsDBgsJCAcDAgQVAggDBBYC AwECHgECF4AACgkQoDSui/t3IH4J+wCfQ5jHdEjCRHj23O/5ttg9r9OIruwAn3103WUITZee e7Sbg12UgcQ5lv7SzsFNBFk3nEQQCACCuTjCjFOUdi5Nm244F+78kLghRcin/awv+IrTcIWF hUpSs1Y91iQQ7KItirz5uwCPlwejSJDQJLIS+QtJHaXDXeV6NI0Uef1hP20+y8qydDiVkv6l IreXjTb7DvksRgJNvCkWtYnlS3mYvQ9NzS9PhyALWbXnH6sIJd2O9lKS1Mrfq+y0IXCP10eS FFGg+Av3IQeFatkJAyju0PPthyTqxSI4lZYuJVPknzgaeuJv/2NccrPvmeDg6Coe7ZIeQ8Yj t0ARxu2xytAkkLCel1Lz1WLmwLstV30g80nkgZf/wr+/BXJW/oIvRlonUkxv+IbBM3dX2OV8 AmRv1ySWPTP7AAMFB/9PQK/VtlNUJvg8GXj9ootzrteGfVZVVT4XBJkfwBcpC/XcPzldjv+3 HYudvpdNK3lLujXeA5fLOH+Z/G9WBc5pFVSMocI71I8bT8lIAzreg0WvkWg5V2WZsUMlnDL9 mpwIGFhlbM3gfDMs7MPMu8YQRFVdUvtSpaAs8OFfGQ0ia3LGZcjA6Ik2+xcqscEJzNH+qh8V m5jjp28yZgaqTaRbg3M/+MTbMpicpZuqF4rnB0AQD12/3BNWDR6bmh+EkYSMcEIpQmBM51qM EKYTQGybRCjpnKHGOxG0rfFY1085mBDZCH5Kx0cl0HVJuQKC+dV2ZY5AqjcKwAxpE75MLFkr wkkEGBECAAkFAlk3nEQCGwwACgkQoDSui/t3IH7nnwCfcJWUDUFKdCsBH/E5d+0ZnMQi+G0A nAuWpQkjM1ASeQwSHEeAWPgskBQL In-Reply-To: <8adbd032-9da2-481d-8d98-30c9a605ec46@citrix.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit On 14.08.2025 21:16, Andrew Cooper wrote: > On 14/08/2025 2:30 pm, Jan Beulich wrote: >> On 08.08.2025 22:23, Andrew Cooper wrote: >>> ... disabled by default. There is a lot of work before FRED can be enabled by >>> default. >>> >>> One part of FRED, the LKGS (Load Kernel GS) instruction, is enumerated >>> separately but is mandatory as FRED disallows the SWAPGS instruction. >>> Therefore, both CPUID bits must be checked. >> See my (further) reply to patch 13 - I think FRED simply ought to depend on >> LKGS. >> >>> @@ -20,6 +22,9 @@ unsigned int __ro_after_init ler_msr; >>> static bool __initdata opt_ler; >>> boolean_param("ler", opt_ler); >>> >>> +int8_t __ro_after_init opt_fred = 0; /* -1 when supported. */ >> I'm a little puzzled by the comment? DYM "once default-enabled"? > > Well, I have this temporary patch > https://gitlab.com/xen-project/hardware/xen-staging/-/commit/70ef6a1178a411a29b7b1745a1112e267ffb6245 > that will turn into a real patch when we enable FRED by default. > > As much as anything else, it was just a TODO. > > >> Then ... >> >>> @@ -305,6 +310,32 @@ void __init traps_init(void) >>> /* Replace early pagefault with real pagefault handler. */ >>> _update_gate_addr_lower(&bsp_idt[X86_EXC_PF], entry_PF); >>> >>> + if ( !cpu_has_fred || !cpu_has_lkgs ) >>> + { >>> + if ( opt_fred ) >> ... this won't work anymore once the initializer is changed. > > Hmm yes.  That wants to be an == 1 check.  Fixed. > >> >>> + printk(XENLOG_WARNING "FRED not available, ignoring\n"); >>> + opt_fred = false; >> Better use 0 here? >> >>> + } >>> + >>> + if ( opt_fred == -1 ) >>> + opt_fred = !pv_shim; >> Imo it would be better to have the initializer be -1 right away, and comment >> out the "!pv_shim" here, until we mean it to be default-enabled. > > It cannot be -1, or Xen will fail spectacularly on any FRED capable > hardware.  Setting to -1 is the point at which FRED becomes security > supported. I guess I'm not following: If it was -1, and if the code here was if ( opt_fred < 0 ) opt_fred = 0 /* !pv_shim */; why would things "fail spectacularly" unless someone passed "fred" on the command line? Jan