linux-kernel - Re: [PATCH v10 33/38] x86/entry: Add fred_entry_from

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Date:   Thu, 21 Sep 2023 15:11:23 +0300
From:   Nikolay Borisov <nik.borisov@...e.com>
To:     Xin Li <xin3.li@...el.com>, linux-doc@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-edac@...r.kernel.org,
        linux-hyperv@...r.kernel.org, kvm@...r.kernel.org,
        xen-devel@...ts.xenproject.org
Cc:     tglx@...utronix.de, mingo@...hat.com, bp@...en8.de,
        dave.hansen@...ux.intel.com, x86@...nel.org, hpa@...or.com,
        luto@...nel.org, pbonzini@...hat.com, seanjc@...gle.com,
        peterz@...radead.org, jgross@...e.com, ravi.v.shankar@...el.com,
        mhiramat@...nel.org, andrew.cooper3@...rix.com,
        jiangshanlai@...il.com
Subject: Re: [PATCH v10 33/38] x86/entry: Add fred_entry_from_kvm() for VMX to
 handle IRQ/NMI



On 14.09.23 г. 7:48 ч., Xin Li wrote:
> In IRQ/NMI induced VM exits, KVM VMX needs to execute the respective
> handlers, which requires the software to create a FRED stack frame,
> and use it to invoke the handlers. Add fred_irq_entry_from_kvm() for
> this job.
> 
> Export fred_entry_from_kvm() because VMX can be compiled as a module.
> 
> Suggested-by: Sean Christopherson <seanjc@...gle.com>
> Tested-by: Shan Kang <shan.kang@...el.com>
> Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
> Signed-off-by: Xin Li <xin3.li@...el.com>
> ---
> 
> Changes since v9:
> * Shove the whole thing into arch/x86/entry/entry_64_fred.S for invoking
>    external_interrupt() and fred_exc_nmi() (Sean Christopherson).
> * Correct and improve a few comments (Sean Christopherson).
> * Merge the two IRQ/NMI asm entries into one as it's fine to invoke
>    noinstr code from regular code (Thomas Gleixner).
> * Setup the long mode and NMI flags in the augmented SS field of FRED
>    stack frame in C instead of asm (Thomas Gleixner).
> * Add UNWIND_HINT_{SAVE,RESTORE} to get rid of the warning: "objtool:
>    asm_fred_entry_from_kvm+0x0: unreachable instruction" (Peter Zijlstra).
> 
> Changes since v8:
> * Add a new macro VMX_DO_FRED_EVENT_IRQOFF for FRED instead of
>    refactoring VMX_DO_EVENT_IRQOFF (Sean Christopherson).
> * Do NOT use a trampoline, just LEA+PUSH the return RIP, PUSH the error
>    code, and jump to the FRED kernel entry point for NMI or call
>    external_interrupt() for IRQs (Sean Christopherson).
> * Call external_interrupt() only when FRED is enabled, and convert the
>    non-FRED handling to external_interrupt() after FRED lands (Sean
>    Christopherson).
> ---
>   arch/x86/entry/entry_64_fred.S | 73 ++++++++++++++++++++++++++++++++++
>   arch/x86/entry/entry_fred.c    | 14 +++++++
>   arch/x86/include/asm/fred.h    | 18 +++++++++
>   3 files changed, 105 insertions(+)
> 
> diff --git a/arch/x86/entry/entry_64_fred.S b/arch/x86/entry/entry_64_fred.S
> index d1c2fc4af8ae..f1088d6f2054 100644
> --- a/arch/x86/entry/entry_64_fred.S
> +++ b/arch/x86/entry/entry_64_fred.S
> @@ -4,7 +4,9 @@
>    */
>   
>   #include <asm/asm.h>
> +#include <asm/export.h>
>   #include <asm/fred.h>
> +#include <asm/segment.h>
>   
>   #include "calling.h"
>   
> @@ -54,3 +56,74 @@ SYM_CODE_START_NOALIGN(asm_fred_entrypoint_kernel)
>   	FRED_EXIT
>   	ERETS
>   SYM_CODE_END(asm_fred_entrypoint_kernel)
> +
> +#if IS_ENABLED(CONFIG_KVM_INTEL)
> +SYM_FUNC_START(asm_fred_entry_from_kvm)
> +	push %rbp
> +	mov %rsp, %rbp

use FRAME_BEGIN/FRAME_END macros to ommit this code if 
CONFIG_FRAME_POINTER is disabled.

> +
> +	UNWIND_HINT_SAVE
> +
> +	/*
> +	 * Don't check the FRED stack level, the call stack leading to this
> +	 * helper is effectively constant and shallow (relatively speaking).
> +	 *
> +	 * Emulate the FRED-defined redzone and stack alignment.
> +	 */
> +	sub $(FRED_CONFIG_REDZONE_AMOUNT << 6), %rsp
> +	and $FRED_STACK_FRAME_RSP_MASK, %rsp
> +
> +	/*
> +	 * Start to push a FRED stack frame, which is always 64 bytes:
> +	 *
> +	 * +--------+-----------------+
> +	 * | Bytes  | Usage           |
> +	 * +--------+-----------------+
> +	 * | 63:56  | Reserved        |
> +	 * | 55:48  | Event Data      |
> +	 * | 47:40  | SS + Event Info |
> +	 * | 39:32  | RSP             |
> +	 * | 31:24  | RFLAGS          |
> +	 * | 23:16  | CS + Aux Info   |
> +	 * |  15:8  | RIP             |
> +	 * |   7:0  | Error Code      |
> +	 * +--------+-----------------+
> +	 */
> +	push $0				/* Reserved, must be 0 */
> +	push $0				/* Event data, 0 for IRQ/NMI */
> +	push %rdi			/* fred_ss handed in by the caller */
> +	push %rbp
> +	pushf
> +	mov $__KERNEL_CS, %rax
> +	push %rax
> +
> +	/*
> +	 * Unlike the IDT event delivery, FRED _always_ pushes an error code
> +	 * after pushing the return RIP, thus the CALL instruction CANNOT be
> +	 * used here to push the return RIP, otherwise there is no chance to
> +	 * push an error code before invoking the IRQ/NMI handler.
> +	 *
> +	 * Use LEA to get the return RIP and push it, then push an error code.
> +	 */
> +	lea 1f(%rip), %rax
> +	push %rax				/* Return RIP */
> +	push $0					/* Error code, 0 for IRQ/NMI */
> +
> +	PUSH_AND_CLEAR_REGS clear_bp=0 unwind_hint=0
> +	movq %rsp, %rdi				/* %rdi -> pt_regs */
> +	call __fred_entry_from_kvm		/* Call the C entry point */
> +	POP_REGS
> +	ERETS
> +1:
> +	/*
> +	 * Objtool doesn't understand what ERETS does, this hint tells it that
> +	 * yes, we'll reach here and with what stack state. A save/restore pair
> +	 * isn't strictly needed, but it's the simplest form.
> +	 */
> +	UNWIND_HINT_RESTORE
> +	pop %rbp

FRAME_END

> +	RET
> +
> +SYM_FUNC_END(asm_fred_entry_from_kvm)
> +EXPORT_SYMBOL_GPL(asm_fred_entry_from_kvm);
> +#endif


<snip>