Re: [Xen-devel] [PATCH v4 3/3] x86/hvm: Implement hvmemul_write() using real mappings

[Paul Durrant <Paul.Durrant@xxxxxxxxxx>]

> -----Original Message-----
> From: Jan Beulich [mailto:JBeulich@xxxxxxxx]
> Sent: 20 September 2017 13:24
> To: Alexandru Isaila <aisaila@xxxxxxxxxxxxxxx>
> Cc: suravee.suthikulpanit@xxxxxxx; Andrew Cooper
> <Andrew.Cooper3@xxxxxxxxxx>; Paul Durrant <Paul.Durrant@xxxxxxxxxx>;
> Wei Liu <wei.liu2@xxxxxxxxxx>; George Dunlap <George.Dunlap@xxxxxxxxxx>;
> Ian Jackson <Ian.Jackson@xxxxxxxxxx>; jun.nakajima@xxxxxxxxx; Kevin Tian
> <kevin.tian@xxxxxxxxx>; sstabellini@xxxxxxxxxx; xen-devel@xxxxxxxxxxxxx;
> boris.ostrovsky@xxxxxxxxxx; konrad.wilk@xxxxxxxxxx; Tim (Xen.org)
> <tim@xxxxxxx>
> Subject: Re: [PATCH v4 3/3] x86/hvm: Implement hvmemul_write() using
> real mappings
> 
> >>> On 20.09.17 at 11:22, <aisaila@xxxxxxxxxxxxxxx> wrote:
> > +static void *hvmemul_map_linear_addr(
> > +    unsigned long linear, unsigned int bytes, uint32_t pfec,
> > +    struct hvm_emulate_ctxt *hvmemul_ctxt)
> > +{
> > +    struct vcpu *curr = current;
> > +    void *err, *mapping;
> > +
> > +    /* First and final gfns which need mapping. */
> > +    unsigned long frame = linear >> PAGE_SHIFT, first = frame;
> > +    unsigned long final = (linear + bytes - !!bytes) >> PAGE_SHIFT;
> > +
> > +    /*
> > +     * mfn points to the next free slot.  All used slots have a page 
> > reference
> > +     * held on them.
> > +     */
> > +    mfn_t *mfn = &hvmemul_ctxt->mfn[0];
> > +
> > +    /*
> > +     * The caller has no legitimate reason for trying a zero-byte write, 
> > but
> > +     * final is calculate to fail safe in release builds.
> > +     *
> > +     * The maximum write size depends on the number of adjacent mfns[]
> which
> > +     * can be vmap()'d, accouting for possible misalignment within the
> region.
> > +     * The higher level emulation callers are responsible for ensuring that
> > +     * mfns[] is large enough for the requested write size.
> > +     */
> > +    if ( bytes == 0 ||
> > +         final - first >= ARRAY_SIZE(hvmemul_ctxt->mfn) )
> > +    {
> > +        ASSERT_UNREACHABLE();
> > +        goto unhandleable;
> > +    }
> > +
> > +    do {
> > +        enum hvm_translation_result res;
> > +        struct page_info *page;
> > +        pagefault_info_t pfinfo;
> > +        p2m_type_t p2mt;
> > +
> > +        /* Error checking.  Confirm that the current slot is clean. */
> > +        ASSERT(mfn_x(*mfn) == 0);
> > +
> > +        res = hvm_translate_get_page(curr, frame << PAGE_SHIFT, true,
> pfec,
> > +                                     &pfinfo, &page, NULL, &p2mt);
> > +
> > +        switch ( res )
> > +        {
> > +        case HVMTRANS_okay:
> > +            break;
> > +
> > +        case HVMTRANS_bad_linear_to_gfn:
> > +            x86_emul_pagefault(pfinfo.ec, pfinfo.linear, &hvmemul_ctxt-
> >ctxt);
> > +            err = ERR_PTR(~X86EMUL_EXCEPTION);
> > +            goto out;
> > +
> > +        case HVMTRANS_bad_gfn_to_mfn:
> > +            err = NULL;
> > +            goto out;
> > +
> > +        case HVMTRANS_gfn_paged_out:
> > +        case HVMTRANS_gfn_shared:
> > +            err = ERR_PTR(~X86EMUL_RETRY);
> > +            goto out;
> > +
> > +        default:
> > +            goto unhandleable;
> > +        }
> > +
> > +        if ( p2m_is_discard_write(p2mt) )
> > +        {
> > +            err = ERR_PTR(~X86EMUL_OKAY);
> > +            goto out;
> > +        }
> > +
> > +        *mfn++ = _mfn(page_to_mfn(page));
> > +
> > +    } while ( ++frame < final );
> 
> Interesting - I had specifically pointed out in a reply to v3 that the
> increment of mfn _cannot_ be moved down here: You're now
> leaking a page ref on the p2m_is_discard_write() error path afaict.

It could be left here if a put_page() is added to the above error path, which 
I'd clearly deluded myself was already there.

  Paul

> 
> Jan


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