Re: [PATCH v5 2/2] xen/domain: rewrite emulation_flags_ok()

[Jan Beulich <jbeulich@xxxxxxxx>]

On 02.06.2025 21:17, dmkhn@xxxxxxxxx wrote:
> From: Denis Mukhin <dmukhin@xxxxxxxx>
> 
> Rewrite emulation_flags_ok() to simplify future modifications.
> 
> No functional change intended.
> 
> Signed-off-by: Denis Mukhin <dmukhin@xxxxxxxx>
> Reviewed-by: Stefano Stabellini <sstabellini@xxxxxxxxxx>
> Reviewed-by: Teddy Astie <teddy.astie@xxxxxxxxxx>
> ---
> Changes since v4:
> - updated commentaries
> - added Teddy's R-b, kept Stefano's R-b
> ---
>  xen/arch/x86/domain.c | 91 ++++++++++++++++++++++++++++++++++---------
>  1 file changed, 73 insertions(+), 18 deletions(-)

Given this diffstat, I wonder what the other x86 maintainers think about
this.

> --- a/xen/arch/x86/domain.c
> +++ b/xen/arch/x86/domain.c
> @@ -743,32 +743,87 @@ int arch_sanitise_domain_config(struct 
> xen_domctl_createdomain *config)
>      return 0;
>  }
>  
> +/*
> + * Verify that the domain's emulation flags resolve to a supported 
> configuration.
> + *
> + * This ensures we only allow a known, safe subset of emulation combinations
> + * (for both functionality and security). Arbitrary mixes are likely to cause
> + * errors (e.g., null pointer dereferences).
> + *
> + * NB: use the internal X86_EMU_XXX symbols, not the public XEN_X86_EMU_XXX
> + * symbols.
> + */
>  static bool emulation_flags_ok(const struct domain *d, uint32_t emflags)
>  {
> +    enum {
> +        CAP_PV          = BIT(0, U),
> +        CAP_HVM         = BIT(1, U),
> +        CAP_HWDOM       = BIT(2, U),
> +        CAP_DOMU        = BIT(3, U),
> +    };
> +    static const struct {
> +        unsigned int caps;
> +        uint32_t min;
> +        uint32_t opt;
> +    } configs[] = {
> +#ifdef CONFIG_PV
> +        /* PV */
> +        {
> +            .caps   = CAP_PV | CAP_DOMU,
> +            .min    = 0,
> +            .opt    = 0,

Why the latter two initializers? Imo adding ones which say nothing else than
what's the default is only enlarging code without much real benefit.

> +        },
> +
> +        /* PV dom0 */
> +        {
> +            .caps   = CAP_PV | CAP_HWDOM,
> +            .min    = X86_EMU_PIT,
> +            .opt    = 0,
> +        },
> +#endif /* #ifdef CONFIG_PV */
> +
> +#ifdef CONFIG_HVM
> +        /* PVH dom0 */
> +        {
> +            .caps   = CAP_HVM | CAP_HWDOM,
> +            .min    = X86_EMU_LAPIC | X86_EMU_IOAPIC | X86_EMU_VPCI,
> +            .opt    = 0,
> +        },
> +
> +        /* HVM domU */
> +        {
> +            .caps   = CAP_HVM | CAP_DOMU,
> +            .min    = X86_EMU_ALL & ~(X86_EMU_VPCI | X86_EMU_USE_PIRQ),
> +            /* HVM PIRQ feature is user-selectable. */
> +            .opt    = X86_EMU_USE_PIRQ,
> +        },
> +
> +        /* PVH domU */
> +        {
> +            .caps   = CAP_HVM | CAP_DOMU,
> +            .min    = X86_EMU_LAPIC,
> +            .opt    = 0,
> +        },
> +#endif /* #ifdef CONFIG_HVM */
> +    };
> +    unsigned int i, caps = is_hardware_domain(d) ? CAP_HWDOM : CAP_DOMU;
> +
> +    if ( is_pv_domain(d) )
> +        caps |= CAP_PV;
> +    else if ( is_hvm_domain(d) )
> +        caps |= CAP_HVM;

There's no 3rd case, so this could be expressed with plain "else", and hence
also with a conditional operator, and hence could also be right in the
initializer. How far to go with those transformations I'm not sure; personally
I'd go all the way, but I'd be okay-ish with just the first of the steps.

Jan