Re: [PATCH v8 11/16] x86/virt/tdx: Designate reserved areas for all TDMRs

[Huang, Kai]

On Tue, 2023-01-10 at 07:19 -0800, Dave Hansen wrote:
> On 1/10/23 03:01, Huang, Kai wrote:
> > On Mon, 2023-01-09 at 17:22 -0800, Dave Hansen wrote:
> > > On 1/9/23 17:19, Huang, Kai wrote:
> > > > > It's probably also worth noting *somewhere* that there's a balance to be
> > > > > had between TDMRs and reserved areas.  A system that is running out of
> > > > > reserved areas in a TDMR could split a TDMR to get more reserved areas.
> > > > > A system that has run out of TDMRs could relatively easily coalesce two
> > > > > adjacent TDMRs (before the PAMTs are allocated) and use a reserved area
> > > > > if there was a gap between them.
> > > > We can add above to the changelog of this patch, or the patch 09 ("x86/virt/tdx:
> > > > Fill out TDMRs to cover all TDX memory regions").  The latter perhaps is better
> > > > since that patch is the first place where the balance of TDMRs and reserved
> > > > areas is related.
> > > > 
> > > > What is your suggestion?
> > > Just put it close to the code that actually hits the problem so the
> > > potential solution is close at hand to whoever hits the problem.
> > > 
> > Sorry to double check: the code which hits the problem is the 'if (idx >=
> > max_reserved_per_tdmr)' check in tdmr_add_rsvd_area(), so I think I can add
> > right before this check?
> 
> Please just hack together how you think it should look and either reply
> with an updated patch, or paste the relevant code snippet in your reply.
>  That'll keep me from having to go chase this code back down.
> 

Thanks for the tip.  How about below?

static int tdmr_add_rsvd_area(struct tdmr_info *tdmr, int *p_idx, u64 addr,
                              u64 size, u16 max_reserved_per_tdmr)
{
        struct tdmr_reserved_area *rsvd_areas = tdmr->reserved_areas;
        int idx = *p_idx;

        /* Reserved area must be 4K aligned in offset and size */
        if (WARN_ON(addr & ~PAGE_MASK || size & ~PAGE_MASK))
                return -EINVAL;

        /*
         * The TDX module supports only limited number of TDMRs and
         * limited number of reserved areas for each TDMR.  There's a
         * balance to be had between TDMRs and reserved areas.  A system
         * that is running out of reserved areas in a TDMR could split a
         * TDMR to get more reserved areas.  A system that has run out
         * of TDMRs could relatively easily coalesce two adjacent TDMRs
         * (before the PAMTs are allocated) and use a reserved area if
         * there was a gap between them.
         */
        if (idx >= max_reserved_per_tdmr) {
                pr_warn("too many reserved areas for TDMR [0x%llx, 0x%llx)\n",
                                tdmr->base, tdmr_end(tdmr));
                return -ENOSPC;
        }

        /*
         * Consume one reserved area per call.  Make no effort to
         * optimize or reduce the number of reserved areas which are
         * consumed by contiguous reserved areas, for instance.
         */
        rsvd_areas[idx].offset = addr - tdmr->base;
        rsvd_areas[idx].size = size;

        *p_idx = idx + 1;

        return 0;
}