Re: [PATCH v4 07/16] x86/virt/tdx: Add tdx_alloc/free_page() helpers

[Nikolay Borisov]

On 2.12.25 г. 0:39 ч., Edgecombe, Rick P wrote:
> On Thu, 2025-11-27 at 18:11 +0200, Nikolay Borisov wrote:
> > > +/* Number PAMT pages to be provided to TDX module per 2M region of PA */
> > > +static int tdx_dpamt_entry_pages(void)
> > > +{
> > > +	if (!tdx_supports_dynamic_pamt(&tdx_sysinfo))
> > > +		return 0;
> > > +
> > > +	return tdx_sysinfo.tdmr.pamt_4k_entry_size * PTRS_PER_PTE /
> > > PAGE_SIZE;
> > > +}
> >
> > Isn't this guaranteed to return 2 always as per the ABI? Can't the
> > allocation of the 2 pages be moved closer to where it's used - in
> > tdh_phymem_pamt_add which will simplify things a bit?
>
> Yea, it could be simpler if it was always guaranteed to be 2 pages. But it was
> my understanding that it would not be a fixed size. Can you point to what docs
> makes you think that?

Looking at the PHYMEM.PAMT.ADD ABI spec the pages being added are always 
put into pair in rdx/r8. So e.g. looking into tdh_phymem_pamt_add rcx is 
set to a 2mb page, and subsequently we have the memcpy which simply sets 
the rdx/r8 input argument registers, no ? Or am I misunderstanding the 
code?
> Another option would be to ask TDX folks to make it fixed, and then require an
> opt-in for it to be expanded later if needed. I would have to check on them on
> the reasoning for it being dynamic sized. I'm not sure if it is *that*
> complicated at this point though. Once there is more than one, the loops becomes
> tempting. And if we loop over 2 we could easily loop over n.