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Thu, 14 Aug 2025 08:00:46 -0700 (PDT) Message-ID: <74a76087-e45e-4d7a-8df3-2711f77eea2a@suse.com> Date: Thu, 14 Aug 2025 17:00:44 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH 18/22] x86/traps: Set MSR_PL0_SSP in load_system_tables() To: Andrew Cooper Cc: =?UTF-8?Q?Roger_Pau_Monn=C3=A9?= , Xen-devel References: <20250808202314.1045968-1-andrew.cooper3@citrix.com> <20250808202314.1045968-19-andrew.cooper3@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: <20250808202314.1045968-19-andrew.cooper3@citrix.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit On 08.08.2025 22:23, Andrew Cooper wrote: > FRED and IDT differ by a Supervisor Token on the base of the shstk. This > means that the value they load into MSR_PL0_SSP differs by 8. > > s3_resume() in particular has logic which is otherwise invariant of FRED mode, > and must not clobber a FRED MSR_PL0_SSP with an IDT one. > > This also simplifies the AP path too. Updating reinit_bsp_stack() is deferred > until later. > > No functional change. > > Signed-off-by: Andrew Cooper I wonder why this was originally done in assembly in the first place, when we aim at reducing tghe assembly code we have. > --- a/xen/arch/x86/boot/x86_64.S > +++ b/xen/arch/x86/boot/x86_64.S > @@ -65,17 +65,11 @@ ENTRY(__high_start) > or $(PRIMARY_SHSTK_SLOT + 1) * PAGE_SIZE - 8, %rdx > > /* > - * Write a new supervisor token. Doesn't matter on boot, but for S3 > - * resume this clears the busy bit. > + * Write a new Supervisor Token. It doesn't matter the first time a > + * CPU boots, but for S3 resume or CPU hot re-add, this clears the > + * busy bit. > */ > wrssq %rdx, (%rdx) > - > - /* Point MSR_PL0_SSP at the token. */ > - mov $MSR_PL0_SSP, %ecx > - mov %edx, %eax > - shr $32, %rdx > - wrmsr > - > setssbsy This is ending up a little odd: The comment says the write is to clear the busy bit, when that's re-set immediately afterwards. Jan