Re: [PATCH for-4.20 4/4] x86/fpu: Split fpu_setup_fpu() in two

[Jan Beulich <jbeulich@xxxxxxxx>]

On 09.07.2024 17:52, Alejandro Vallejo wrote:
> It's doing too many things at once and there's no clear way of defining what
> it's meant to do. This patch splits the function in two.
> 
>   1. A reset function, parameterized by the FCW value. FCW_RESET means to 
> reset
>      the state to power-on reset values, while FCW_DEFAULT means to reset to 
> the
>      default values present during vCPU creation.
>   2. A x87/SSE state loader (equivalent to the old function when it took a 
> data
>      pointer).
> 
> Signed-off-by: Alejandro Vallejo <alejandro.vallejo@xxxxxxxxx>
> ---
> I'm still not sure what the old function tries to do. The state we start vCPUs
> in is _similar_ to the after-finit, but it's not quite (`ftw` is not -1). I 
> went
> for the "let's not deviate too much from previous behaviour", but maybe we did
> intend for vCPUs to start as if `finit` had just been executed?

A relevant aspect here may be that what FSXR and XSAVE area have is only an
abridged form of the tag word, being only 8 bits in size. 0x00 there is
equivalent to FTW=0xffff (all st(<N>) empty). That's not quite correct for
the reset case indeed, where FTW=0x5555 (i.e. all st(<N>) zero, requiring
the abridged form to hold 0xff instead). While no-one has reported issues
there so far, I think it wouldn't be inappropriate to correct this.

> --- a/xen/arch/x86/hvm/hvm.c
> +++ b/xen/arch/x86/hvm/hvm.c
> @@ -1162,10 +1162,17 @@ static int cf_check hvm_load_cpu_ctxt(struct domain 
> *d, hvm_domain_context_t *h)
>      seg.attr = ctxt.ldtr_arbytes;
>      hvm_set_segment_register(v, x86_seg_ldtr, &seg);
>  
> -    /* Cover xsave-absent save file restoration on xsave-capable host. */
> -    vcpu_setup_fpu(v, xsave_enabled(v) ? NULL : v->arch.xsave_area,
> -                   ctxt.flags & XEN_X86_FPU_INITIALISED ? ctxt.fpu_regs : 
> NULL,
> -                   FCW_RESET);
> +    /*
> +     * On Xen 4.1 and later the FPU state is restored on a later HVM 
> context, so
> +     * what we're doing here is initialising the FPU state for guests from 
> even
> +     * older versions of Xen. In general such guests only use legacy x87/SSE
> +     * state, and if they did use XSAVE then our best-effort strategy is to 
> make
> +     * an XSAVE header for x87 and SSE hoping that's good enough.
> +     */
> +    if ( ctxt.flags & XEN_X86_FPU_INITIALISED )
> +        vcpu_setup_fpu(v, &ctxt.fpu_regs);
> +    else
> +        vcpu_reset_fpu(v, FCW_RESET);

I'm struggling with the use of "later" in the comment. What exactly is that
meant to express? Fundamentally the XSAVE data is fully backwards compatible
with the FXSR one, I think, so the mentioning of "best-effort" isn't quite
clear to me either.

> --- a/xen/arch/x86/i387.c
> +++ b/xen/arch/x86/i387.c
> @@ -310,41 +310,25 @@ int vcpu_init_fpu(struct vcpu *v)
>      return xstate_alloc_save_area(v);
>  }
>  
> -void vcpu_setup_fpu(struct vcpu *v, struct xsave_struct *xsave_area,
> -                    const void *data, unsigned int fcw_default)
> +void vcpu_reset_fpu(struct vcpu *v, uint16_t fcw)
>  {
> -    fpusse_t *fpu_sse = &v->arch.xsave_area->fpu_sse;
> -
> -    ASSERT(!xsave_area || xsave_area == v->arch.xsave_area);
> -
> -    v->fpu_initialised = !!data;
> -
> -    if ( data )
> -    {
> -        memcpy(fpu_sse, data, sizeof(*fpu_sse));
> -        if ( xsave_area )
> -            xsave_area->xsave_hdr.xstate_bv = XSTATE_FP_SSE;
> -    }
> -    else if ( xsave_area && fcw_default == FCW_DEFAULT )
> -    {
> -        xsave_area->xsave_hdr.xstate_bv = 0;
> -        fpu_sse->mxcsr = MXCSR_DEFAULT;
> -    }
> -    else
> -    {
> -        memset(fpu_sse, 0, sizeof(*fpu_sse));
> -        fpu_sse->fcw = fcw_default;
> -        fpu_sse->mxcsr = MXCSR_DEFAULT;
> -        if ( v->arch.xsave_area )
> -        {
> -            v->arch.xsave_area->xsave_hdr.xstate_bv &= ~XSTATE_FP_SSE;
> -            if ( fcw_default != FCW_DEFAULT )
> -                v->arch.xsave_area->xsave_hdr.xstate_bv |= X86_XCR0_X87;
> -        }
> -    }
> +    v->fpu_initialised = false;
> +    *v->arch.xsave_area = (struct xsave_struct) {
> +        .fpu_sse = {
> +            .mxcsr = MXCSR_DEFAULT,
> +            .fcw = fcw,
> +        },
> +        .xsave_hdr.xstate_bv = fcw == FCW_RESET ? X86_XCR0_X87 : 0,
> +    };
> +}

Old code checked against FCW_DEFAULT uniformly. You switching to checking
against FCW_RESET is no functional change only because all callers pass
either of the two values. I wonder whether the new function's parameter
wouldn't want to be a boolean (reset vs init).

> -    if ( xsave_area )
> -        xsave_area->xsave_hdr.xcomp_bv = 0;
> +void vcpu_setup_fpu(struct vcpu *v, const void *data)
> +{
> +    v->fpu_initialised = true;
> +    *v->arch.xsave_area = (struct xsave_struct) {
> +        .fpu_sse = *(fpusse_t*)data,

First of all please never cast away const. See Misra rule 11.8. And then
a nit again: Blank ahead of the latter of the two *-s, please.

> --- a/xen/arch/x86/include/asm/i387.h
> +++ b/xen/arch/x86/include/asm/i387.h
> @@ -31,10 +31,29 @@ void vcpu_restore_fpu_nonlazy(struct vcpu *v, bool 
> need_stts);
>  void vcpu_restore_fpu_lazy(struct vcpu *v);
>  void vcpu_save_fpu(struct vcpu *v);
>  void save_fpu_enable(void);
> -
>  int vcpu_init_fpu(struct vcpu *v);
> -struct xsave_struct;
> -void vcpu_setup_fpu(struct vcpu *v, struct xsave_struct *xsave_area,
> -                    const void *data, unsigned int fcw_default);
>  void vcpu_destroy_fpu(struct vcpu *v);
> +
> +/*
> + * Restore `v`'s FPU to known values
> + *
> + * If fcw == FCW_RESET, then the reset state is power-on RESET.
> + *
> + * Otherwise `mxcsr` is set to `MXCSR_DEFAULT`, `fcw` is overriden with the
> + * `fcw` argument and everything else is zeroed out.

Backticks are used for two different purposes here, which I'm afraid is
confusing. You want to make it easy to tell function arguments from other
entities, imo.

> + * @param v   vCPU containing the FPU
> + * @param fcw Intended FPU Control Word
> + */
> +void vcpu_reset_fpu(struct vcpu *v, uint16_t fcw);
> +
> +/*
> + * Load x87/SSE state into `v`'s FPU

Applicable here then as well.

Jan