Re: [PATCH v3 19/22] x86emul: support TILELOADD{,T1} and TILESTORE

[Paul Durrant <xadimgnik@xxxxxxxxx>]

On 22/04/2021 16:11, Jan Beulich wrote:
> On 22.04.2021 17:06, Jan Beulich wrote:
> > On 22.04.2021 16:55, Jan Beulich wrote:
> > > +        do {
> > > +            /* Limit rows to just as many to cover the next one to access. */
> > > +            cfg->start_row = i;
> > > +            cfg->rows[modrm_reg] = i + 1;
> > > +            write_tilecfg(cfg);
> > > +
> > > +            if ( vex.pfx != vex_f3 )
> > > +                rc = ops->read(ea.mem.seg,
> > > +                               truncate_ea(ea.mem.off + i * ea.val),
> > > +                               row, cfg->colsb[modrm_reg], ctxt);
> > > +
> > > +            invoke_stub("", "", "=m" (dummy) : "a" (row));
> > > +
> > > +            if ( vex.pfx == vex_f3 )
> > > +                rc = ops->write(ea.mem.seg,
> > > +                                truncate_ea(ea.mem.off + i * ea.val),
> > > +                                row, cfg->colsb[modrm_reg], ctxt);
> > > +        } while ( rc == X86EMUL_OKAY && ++i < n );
> > in principle tiles could have rows larger than 64 bytes without any
> > separate CPUID feature flag qualifying this. struct hvm_mmio_cache,
> > otoh, is having a fixed-size 64-byte buffer right now. Therefore I'm
> > wondering whether we'd want to switch to dynamically allocating that
> > to the minimum of 64 bytes and the size of a tile row, just as a
> > precautionary measure.
> Actually, as it occurred to me only after sending, enlarging tile size
> would under almost all circumstances require a new XSTATE component,
> which we'd need to enable first. I consider it less likely that they'd
> permit a wider range of layouts without increasing tile size. But we
> might still want to play safe.
I guess on-demand reallocation to a larger size would be fine. Certainly 
we want to be sure we don't overflow.
  Paul

> Jan