Re: [Xen-devel] [PATCH v2] x86/HVM: correct hvmemul_map_linear_addr() for multi-page case

[Paul Durrant <Paul.Durrant@xxxxxxxxxx>]

> -----Original Message-----
> From: Jan Beulich [mailto:JBeulich@xxxxxxxx]
> Sent: 13 September 2018 11:13
> To: xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxxx>
> Cc: Andrew Cooper <Andrew.Cooper3@xxxxxxxxxx>; Paul Durrant
> <Paul.Durrant@xxxxxxxxxx>
> Subject: [PATCH v2] x86/HVM: correct hvmemul_map_linear_addr() for multi-
> page case
> 
> The function does two translations in one go for a single guest access.
> Any failure of the first translation step (guest linear -> guest
> physical), resulting in #PF, ought to take precedence over any failure
> of the second step (guest physical -> host physical). Bail out of the
> loop early solely when translation produces HVMTRANS_bad_linear_to_gfn,
> and record the most relevant of perhaps multiple different errors
> otherwise. (The choice of ZERO_BLOCK_PTR as sentinel is arbitrary.)
> 
> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
> ---
> v2: Add comment (mapping table) and adjust update_map_err()
>     accordingly.
> 
> --- a/xen/arch/x86/hvm/emulate.c
> +++ b/xen/arch/x86/hvm/emulate.c
> @@ -532,6 +532,36 @@ static int hvmemul_do_mmio_addr(paddr_t
>  }
> 
>  /*
> + * Intended mapping, implemented without table lookup:
> + *
> + * -----------------------------------------
> + * | \ new |       |       |       |       |
> + * |   \   | OKAY  | NULL  | RETRY | UNHND |
> + * | err \ |       |       |       |       |
> + * -----------------------------------------
> + * | OKAY  | OKAY  | NULL  | RETRY | UNHND |
> + * -----------------------------------------
> + * | NULL  | NULL  | NULL  | RETRY | UNHND |
> + * -----------------------------------------
> + * | RETRY | RETRY | RETRY | RETRY | UNHND |
> + * -----------------------------------------
> + * | UNHND | UNHND | UNHND | UNHND | UNHND |
> + * -----------------------------------------
> + */
> +static void *update_map_err(void *err, void *new)
> +{
> +    if ( err == ZERO_BLOCK_PTR || err == ERR_PTR(~X86EMUL_OKAY) ||
> +         new == ERR_PTR(~X86EMUL_UNHANDLEABLE) )
> +        return new;
> +
> +    if ( new == ERR_PTR(~X86EMUL_OKAY) ||
> +         err == ERR_PTR(~X86EMUL_UNHANDLEABLE) )
> +        return err;
> +
> +    return err ?: new;

Took a while to check but that logic does match the table :-)

> +}
> +
> +/*
>   * Map the frame(s) covering an individual linear access, for writeable
>   * access.  May return NULL for MMIO, or ERR_PTR(~X86EMUL_*) for other
> errors
>   * including ERR_PTR(~X86EMUL_OKAY) for write-discard mappings.
> @@ -544,7 +574,7 @@ static void *hvmemul_map_linear_addr(
>      struct hvm_emulate_ctxt *hvmemul_ctxt)
>  {
>      struct vcpu *curr = current;
> -    void *err, *mapping;
> +    void *err = ZERO_BLOCK_PTR, *mapping;

Given the revised logic, can't you just start *err with the value 
ERR_PTR(~X86EMUL_OKAY) now? You can then lose the extra test in the first if of 
update_map_err().

  Paul

>      unsigned int nr_frames = ((linear + bytes - !!bytes) >> PAGE_SHIFT) -
>          (linear >> PAGE_SHIFT) + 1;
>      unsigned int i;
> @@ -600,27 +630,28 @@ static void *hvmemul_map_linear_addr(
>              goto out;
> 
>          case HVMTRANS_bad_gfn_to_mfn:
> -            err = NULL;
> -            goto out;
> +            err = update_map_err(err, NULL);
> +            continue;
> 
>          case HVMTRANS_gfn_paged_out:
>          case HVMTRANS_gfn_shared:
> -            err = ERR_PTR(~X86EMUL_RETRY);
> -            goto out;
> +            err = update_map_err(err, ERR_PTR(~X86EMUL_RETRY));
> +            continue;
> 
>          default:
> -            goto unhandleable;
> +            err = update_map_err(err, ERR_PTR(~X86EMUL_UNHANDLEABLE));
> +            continue;
>          }
> 
>          *mfn++ = page_to_mfn(page);
> 
>          if ( p2m_is_discard_write(p2mt) )
> -        {
> -            err = ERR_PTR(~X86EMUL_OKAY);
> -            goto out;
> -        }
> +            err = update_map_err(err, ERR_PTR(~X86EMUL_OKAY));
>      }
> 
> +    if ( err != ZERO_BLOCK_PTR )
> +        goto out;
> +
>      /* Entire access within a single frame? */
>      if ( nr_frames == 1 )
>          mapping = map_domain_page(hvmemul_ctxt->mfn[0]);
> @@ -639,6 +670,7 @@ static void *hvmemul_map_linear_addr(
>      return mapping + (linear & ~PAGE_MASK);
> 
>   unhandleable:
> +    ASSERT(err == ZERO_BLOCK_PTR);
>      err = ERR_PTR(~X86EMUL_UNHANDLEABLE);
> 
>   out:
> 
> 
> 


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