Re: [Xen-devel] [PATCH v3 05/17] xen/arm64: Implement a fast path for handling SMCCC_ARCH_WORKAROUND_1

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

On Thu, 15 Feb 2018, Julien Grall wrote:
> The function SMCCC_ARCH_WORKAROUND_1 will be called by the guest for
> hardening the branch predictor. So we want the handling to be as fast as
> possible.
> 
> As the mitigation is applied on every guest exit, we can check for the
> call before saving all the context and return very early.
> 
> For now, only provide a fast path for HVC64 call. Because the code rely
> on 2 registers, x0 and x1 are saved in advance.
> 
> Signed-off-by: Julien Grall <julien.grall@xxxxxxx>
> Reviewed-by: Volodymyr Babchuk <volodymyr.babchuk@xxxxxxxx>

Nice!

Reviewed-by: Stefano Stabellini <sstabellini@xxxxxxxxxx>


> ---
>     guest_sync only handle 64-bit guest, so I have only implemented the
>     64-bit side for now. We can discuss whether it is useful to
>     implement it for 32-bit guests.
> 
>     We could also consider to implement the fast path for SMC64,
>     althought a guest should always use HVC.
> 
>     Changes in v2:
>         - Add Volodymyr's reviewed-by
> ---
>  xen/arch/arm/arm64/entry.S      | 56 
> +++++++++++++++++++++++++++++++++++++++--
>  xen/include/asm-arm/processor.h |  2 ++
>  2 files changed, 56 insertions(+), 2 deletions(-)
> 
> diff --git a/xen/arch/arm/arm64/entry.S b/xen/arch/arm/arm64/entry.S
> index 6d99e46f0f..67f96d518f 100644
> --- a/xen/arch/arm/arm64/entry.S
> +++ b/xen/arch/arm/arm64/entry.S
> @@ -1,6 +1,7 @@
>  #include <asm/asm_defns.h>
>  #include <asm/regs.h>
>  #include <asm/alternative.h>
> +#include <asm/smccc.h>
>  #include <public/xen.h>
>  
>  /*
> @@ -90,8 +91,12 @@ lr      .req    x30             /* link register */
>          .endm
>  /*
>   * Save state on entry to hypervisor, restore on exit
> + *
> + * save_x0_x1: Does the macro needs to save x0/x1 (default 1). If 0,
> + * we rely on the on x0/x1 to have been saved at the correct position on
> + * the stack before.
>   */
> -        .macro  entry, hyp, compat
> +        .macro  entry, hyp, compat, save_x0_x1=1
>          sub     sp, sp, #(UREGS_SPSR_el1 - UREGS_LR) /* CPSR, PC, SP, LR */
>          push    x28, x29
>          push    x26, x27
> @@ -107,7 +112,16 @@ lr      .req    x30             /* link register */
>          push    x6, x7
>          push    x4, x5
>          push    x2, x3
> +        /*
> +         * The caller may already have saved x0/x1 on the stack at the
> +         * correct address and corrupt them with another value. Only
> +         * save them if save_x0_x1 == 1.
> +         */
> +        .if \save_x0_x1 == 1
>          push    x0, x1
> +        .else
> +        sub     sp, sp, #16
> +        .endif
>  
>          .if \hyp == 1        /* Hypervisor mode */
>  
> @@ -200,7 +214,45 @@ hyp_irq:
>          exit    hyp=1
>  
>  guest_sync:
> -        entry   hyp=0, compat=0
> +        /*
> +         * Save x0, x1 in advance
> +         */
> +        stp     x0, x1, [sp, #-(UREGS_kernel_sizeof - UREGS_X0)]
> +
> +        /*
> +         * x1 is used because x0 may contain the function identifier.
> +         * This avoids to restore x0 from the stack.
> +         */
> +        mrs     x1, esr_el2
> +        lsr     x1, x1, #HSR_EC_SHIFT           /* x1 = ESR_EL2.EC */
> +        cmp     x1, #HSR_EC_HVC64
> +        b.ne    1f                              /* Not a HVC skip fastpath. 
> */
> +
> +        mrs     x1, esr_el2
> +        and     x1, x1, #0xffff                 /* Check the immediate 
> [0:16] */
> +        cbnz    x1, 1f                          /* should be 0 for HVC #0 */
> +
> +        /*
> +         * Fastest path possible for ARM_SMCCC_ARCH_WORKAROUND_1.
> +         * The workaround has already been applied on the exception
> +         * entry from the guest, so let's quickly get back to the guest.
> +         */
> +        eor     w0, w0, #ARM_SMCCC_ARCH_WORKAROUND_1_FID
> +        cbnz    w0, 1f
> +
> +        /*
> +         * Clobber both x0 and x1 to prevent leakage. Note that thanks
> +         * the eor, x0 = 0.
> +         */
> +        mov     x1, x0
> +        eret
> +
> +1:
> +        /*
> +         * x0/x1 may have been scratch by the fast path above, so avoid
> +         * to save them.
> +         */
> +        entry   hyp=0, compat=0, save_x0_x1=0
>          /*
>           * The vSError will be checked while 
> SKIP_SYNCHRONIZE_SERROR_ENTRY_EXIT
>           * is not set. If a vSError took place, the initial exception will be
> diff --git a/xen/include/asm-arm/processor.h b/xen/include/asm-arm/processor.h
> index c0f79d0093..222a02dd99 100644
> --- a/xen/include/asm-arm/processor.h
> +++ b/xen/include/asm-arm/processor.h
> @@ -306,6 +306,8 @@
>  #define HDCR_TPM        (_AC(1,U)<<6)           /* Trap Performance Monitors 
> accesses */
>  #define HDCR_TPMCR      (_AC(1,U)<<5)           /* Trap PMCR accesses */
>  
> +#define HSR_EC_SHIFT                26
> +
>  #define HSR_EC_UNKNOWN              0x00
>  #define HSR_EC_WFI_WFE              0x01
>  #define HSR_EC_CP15_32              0x03
> -- 
> 2.11.0
> 

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