Re: [Xen-devel] [PATCH v6 02/16] x86/hvm: remove multiple open coded 'chunking' loops

[Paul Durrant <Paul.Durrant@xxxxxxxxxx>]

> -----Original Message-----
> From: xen-devel-bounces@xxxxxxxxxxxxx [mailto:xen-devel-
> bounces@xxxxxxxxxxxxx] On Behalf Of Jan Beulich
> Sent: 08 July 2015 16:53
> To: Andrew Cooper; Paul Durrant
> Cc: xen-devel@xxxxxxxxxxxxxxxxxxxx; Keir (Xen.org)
> Subject: Re: [Xen-devel] [PATCH v6 02/16] x86/hvm: remove multiple open
> coded 'chunking' loops
> 
> >>> On 03.07.15 at 18:25, <paul.durrant@xxxxxxxxxx> wrote:
> > +static int hvmemul_linear_mmio_access(
> > +    unsigned long gla, unsigned int size, uint8_t dir, uint8_t *buffer,
> > +    uint32_t pfec, struct hvm_emulate_ctxt *hvmemul_ctxt, bool_t
> known_gpfn)
> > +{
> > +    struct hvm_vcpu_io *vio = &current->arch.hvm_vcpu.hvm_io;
> > +    unsigned long offset = gla & ~PAGE_MASK;
> > +    unsigned int chunk;
> > +    paddr_t gpa;
> > +    unsigned long one_rep = 1;
> > +    int rc;
> > +
> > +    chunk = min_t(unsigned int, size, PAGE_SIZE - offset);
> > +
> > +    if ( known_gpfn )
> > +        gpa = pfn_to_paddr(vio->mmio_gpfn) | offset;
> > +    else
> > +    {
> > +        rc = hvmemul_linear_to_phys(gla, &gpa, chunk, &one_rep, pfec,
> > +                                    hvmemul_ctxt);
> > +        if ( rc != X86EMUL_OKAY )
> > +            return rc;
> > +    }
> > +
> > +    for ( ;; )
> > +    {
> > +        rc = hvmemul_phys_mmio_access(gpa, chunk, dir, buffer);
> > +        if ( rc != X86EMUL_OKAY )
> > +            break;
> > +
> > +        gla += chunk;
> > +        buffer += chunk;
> > +        size -= chunk;
> > +
> > +        if ( size == 0 )
> > +            break;
> > +
> > +        ASSERT((gla & ~PAGE_MASK) == 0);
> 
> Does this really matter for the code below?
> 
> > +        chunk = min_t(unsigned int, size, PAGE_SIZE);
> 
> Iirc Andrew had asked for this, but I still don't see why: "size" is the
> width of an instruction operand, and hence won't even come close
> to PAGE_SIZE.
> 

I'll let Andrew answer that one. I've already ping-ponged once on this.

I guess it makes it safer if someone were to try to use this function for 
another purpose in future?

  Paul

> > +        rc = hvmemul_linear_to_phys(gla, &gpa, chunk, &one_rep, pfec,
> > +                                    hvmemul_ctxt);
> 
> The fact that &one_rep is being passed here also makes clear that
> we can't be dealing with an <operand-width> * <reps> situation.
> And if we were, "size" couldn't validly be "unsigned int".
> 
> Jan
> 
> 
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