Re: [PATCH v2 2/3] xen/pv: refactor msr access functions to support safe and unsafe accesses

[Jan Beulich <jbeulich@xxxxxxxx>]

On 04.10.2022 10:43, Juergen Gross wrote:
> Refactor and rename xen_read_msr_safe() and xen_write_msr_safe() to
> support both cases of MSR accesses, safe ones and potentially GP-fault
> generating ones.
> 
> This will prepare to no longer swallow GPs silently in xen_read_msr()
> and xen_write_msr().
> 
> Signed-off-by: Juergen Gross <jgross@xxxxxxxx>

Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>

Albeit ...

> @@ -933,23 +937,39 @@ static u64 xen_read_msr_safe(unsigned int msr, int *err)
>       return val;
>  }
>  
> -static int xen_write_msr_safe(unsigned int msr, unsigned low, unsigned high)
> +static void set_seg(unsigned int which, unsigned int low, unsigned int high,
> +                 int *err)
>  {
> -     int ret;
> -     unsigned int which;
> -     u64 base;
> +     u64 base = ((u64)high << 32) | low;
> +
> +     if (HYPERVISOR_set_segment_base(which, base) == 0)
> +             return;
>  
> -     ret = 0;
> +     if (err)
> +             *err = -EIO;

... I don't see a good reason to override the error code handed to us
by the hypervisor here; I do realize though that this ...

> +     else
> +             WARN(1, "Xen set_segment_base(%u, %llx) failed\n", which, base);
> +}
>  
> +/*
> + * Support write_msr_safe() and write_msr() semantics.
> + * With err == NULL write_msr() semantics are selected.
> + * Supplying an err pointer requires err to be pre-initialized with 0.
> + */
> +static void xen_do_write_msr(unsigned int msr, unsigned int low,
> +                          unsigned int high, int *err)
> +{
>       switch (msr) {
> -     case MSR_FS_BASE:               which = SEGBASE_FS; goto set;
> -     case MSR_KERNEL_GS_BASE:        which = SEGBASE_GS_USER; goto set;
> -     case MSR_GS_BASE:               which = SEGBASE_GS_KERNEL; goto set;
> -
> -     set:
> -             base = ((u64)high << 32) | low;
> -             if (HYPERVISOR_set_segment_base(which, base) != 0)
> -                     ret = -EIO;

... was this way before.

Jan