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Message-ID: <23568229ee014c9f61c27ee310573782fa0d1e8d.camel@intel.com>
Date: Fri, 19 Jan 2024 09:49:43 +0000
From: "Huang, Kai" <kai.huang@...el.com>
To: "kirill.shutemov@...ux.intel.com" <kirill.shutemov@...ux.intel.com>,
"tglx@...utronix.de" <tglx@...utronix.de>, "mingo@...hat.com"
<mingo@...hat.com>, "bp@...en8.de" <bp@...en8.de>,
"dave.hansen@...ux.intel.com" <dave.hansen@...ux.intel.com>
CC: "hpa@...or.com" <hpa@...or.com>, "linux-kernel@...r.kernel.org"
<linux-kernel@...r.kernel.org>, "ak@...ux.intel.com" <ak@...ux.intel.com>,
"x86@...nel.org" <x86@...nel.org>, "seanjc@...gle.com" <seanjc@...gle.com>
Subject: Re: [PATCHv3] x86/trampoline: Bypass compat mode in
trampoline_start64() if not needed
On Tue, 2024-01-09 at 01:16 +0300, Kirill A. Shutemov wrote:
> The trampoline_start64() vector is used when a secondary CPU starts in
> 64-bit mode. The current implementation directly enters compatibility
> mode. It is necessary to disable paging and re-enable it in the correct
> paging mode: either 4- or 5-level, depending on the configuration.
>
> The X86S[1] ISA does not support compatibility mode in ring 0, and
> paging cannot be disabled.
>
> The trampoline_start64() function is reworked to only enter compatibility
> mode if it is necessary to change the paging mode. If the CPU is already
> in the desired paging mode, it will proceed in long mode.
>
> This change will allow a secondary CPU to boot on an X86S machine as
> long as the CPU is already in the correct paging mode.
>
> In the future, there will be a mechanism to switch between paging modes
> without disabling paging.
>
> [1] https://www.intel.com/content/www/us/en/developer/articles/technical/envisioning-future-simplified-architecture.html
>
> Signed-off-by: Kirill A. Shutemov <kirill.shutemov@...ux.intel.com>
> Reviewed-by: Andi Kleen <ak@...ux.intel.com>
> Cc: Sean Christopherson <seanjc@...gle.com>
>
Reviewed-by: Kai Huang <kai.huang@...el.com>
> ---
> v3:
> - tr_cr4 is 32-bit, use 32-bit XOR to access it (Sean).
> - Use 32-bit TEST instead of AND to check if LA57 different between
> CR4 and tr_cr4 (Sean).
> v2:
> - Fix build with GCC;
> ---
> arch/x86/realmode/rm/trampoline_64.S | 31 +++++++++++++++++++++++++++-
> 1 file changed, 30 insertions(+), 1 deletion(-)
>
> diff --git a/arch/x86/realmode/rm/trampoline_64.S b/arch/x86/realmode/rm/trampoline_64.S
> index c9f76fae902e..608f108cba7d 100644
> --- a/arch/x86/realmode/rm/trampoline_64.S
> +++ b/arch/x86/realmode/rm/trampoline_64.S
> @@ -37,13 +37,15 @@
> .text
> .code16
>
> -.macro LOCK_AND_LOAD_REALMODE_ESP lock_pa=0
> +.macro LOCK_AND_LOAD_REALMODE_ESP lock_pa=0 lock_rip=0
> /*
> * Make sure only one CPU fiddles with the realmode stack
> */
> .Llock_rm\@:
> .if \lock_pa
> lock btsl $0, pa_tr_lock
> + .elseif \lock_rip
> + lock btsl $0, tr_lock(%rip)
> .else
> lock btsl $0, tr_lock
> .endif
> @@ -220,6 +222,33 @@ SYM_CODE_START(trampoline_start64)
> lidt tr_idt(%rip)
> lgdt tr_gdt64(%rip)
>
> + /* Check if paging mode has to be changed */
> + movq %cr4, %rax
> + xorl tr_cr4(%rip), %eax
> + testl $X86_CR4_LA57, %eax
> + jnz .L_switch_paging
> +
> + /* Paging mode is correct proceed in 64-bit mode */
> +
> + LOCK_AND_LOAD_REALMODE_ESP lock_rip=1
> +
> + movw $__KERNEL_DS, %dx
> + movl %edx, %ss
> + addl $pa_real_mode_base, %esp
> + movl %edx, %ds
> + movl %edx, %es
> + movl %edx, %fs
> + movl %edx, %gs
> +
> + movl $pa_trampoline_pgd, %eax
> + movq %rax, %cr3
> +
> + jmpq *tr_start(%rip)
> +.L_switch_paging:
> + /*
> + * To switch between 4- and 5-level paging modes, it is necessary
> + * to disable paging. This must be done in the compatibility mode.
> + */
> ljmpl *tr_compat(%rip)
> SYM_CODE_END(trampoline_start64)
>
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