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Re: [PATCH v3 14/22] x86/boot: choose AP stack based on APIC ID
- To: Sergii Dmytruk <sergii.dmytruk@xxxxxxxxx>
- From: Jan Beulich <jbeulich@xxxxxxxx>
- Date: Thu, 22 Jan 2026 16:52:36 +0100
- Autocrypt: addr=jbeulich@xxxxxxxx; 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
- Cc: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>, Roger Pau Monné <roger.pau@xxxxxxxxxx>, xen-devel@xxxxxxxxxxxxxxxxxxxx, trenchboot-devel@xxxxxxxxxxxxxxxx
- Delivery-date: Thu, 22 Jan 2026 15:53:00 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On 30.05.2025 15:17, Sergii Dmytruk wrote:
> --- a/xen/arch/x86/boot/trampoline.S
> +++ b/xen/arch/x86/boot/trampoline.S
> @@ -71,6 +71,27 @@ trampoline_protmode_entry:
> mov $X86_CR4_PAE,%ecx
> mov %ecx,%cr4
>
> + /*
> + * Get APIC ID while we're in non-paged mode. Start by checking if
> + * x2APIC is enabled.
> + */
> + mov $MSR_APIC_BASE, %ecx
> + rdmsr
> + test $APIC_BASE_EXTD, %eax
> + jnz .Lx2apic
> +
> + /* Not x2APIC, read from MMIO */
> + and $APIC_BASE_ADDR_MASK, %eax
> + mov APIC_ID(%eax), %esp
> + shr $24, %esp
I have to admit that I'm rather hesitant towards seeing %esp used like this.
> --- a/xen/arch/x86/boot/x86_64.S
> +++ b/xen/arch/x86/boot/x86_64.S
> @@ -15,7 +15,33 @@ ENTRY(__high_start)
> mov $XEN_MINIMAL_CR4,%rcx
> mov %rcx,%cr4
>
> - mov stack_start(%rip),%rsp
> + test %ebx,%ebx
> + cmovz stack_start(%rip), %rsp
> + jz .L_stack_set
> +
> + /* APs only: get stack base from APIC ID saved in %esp. */
> + mov $-1, %rax
Here and below 32-bit insn would do fine. However, ...
> + lea x86_cpu_to_apicid(%rip), %rcx
> +1:
> + add $1, %rax
> + cmp $NR_CPUS, %eax
> + jb 2f
> + hlt
> +2:
> + cmp %esp, (%rcx, %rax, 4)
> + jne 1b
... can't all of this be a simple REPNE SCASL?
As to the upper bound of NR_CPUS, do we really need to look this far?
> + /* %eax is now Xen CPU index. */
> + lea stack_base(%rip), %rcx
> + mov (%rcx, %rax, 8), %rsp
> +
> + test %rsp,%rsp
Nit: Blank after comma please.
Jan
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