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Subject: RE: [PATCH 2/6] x86/tdx: Retry TDVMCALL_MAP_GPA() when needed
Thread-Topic: [PATCH 2/6] x86/tdx: Retry TDVMCALL_MAP_GPA() when needed
Thread-Index: AQHY/uuA5M0FGuMnSkmqWvcQ39Wr/g==
Date:   Wed, 23 Nov 2022 03:27:25 +0000
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> From: Kirill A. Shutemov <kirill@shutemov.name>
> Sent: Monday, November 21, 2022 4:01 PM
> [...]
> On Mon, Nov 21, 2022 at 11:51:47AM -0800, Dexuan Cui wrote:
> [...]
> I'm not convinced it deserves a separate helper for one user.
> Does it look that ugly if tdx_map_gpa() uses __tdx_hypercall() directly?

Will use __tdx_hypercall() directly.
=20
> >  /* Called from __tdx_hypercall() for unrecoverable failure */
> >  void __tdx_hypercall_failed(void)
> >  {
> > @@ -691,6 +712,43 @@ static bool try_accept_one(phys_addr_t *start,
> unsigned long len,
> >  	return true;
> >  }
> >
> > +/*
> > + * Notify the VMM about page mapping conversion. More info about ABI
> > + * can be found in TDX Guest-Host-Communication Interface (GHCI),
> > + * section "TDG.VP.VMCALL<MapGPA>"
> > + */
> > +static bool tdx_map_gpa(phys_addr_t start, phys_addr_t end, bool enc)
> > +{
> > +	u64 ret, r11;
> > +
> > +	while (1) {
>=20
> Endless? Maybe an upper limit if no progress?

I'll add a max count of 1000, which should be far more than enough.

> > +		ret =3D _tdx_hypercall_output_r11(TDVMCALL_MAP_GPA, start,
> > +						end - start, 0, 0, &r11);
> > +		if (!ret)
> > +			break;
> > +
> > +		if (ret !=3D TDVMCALL_STATUS_RETRY)
> > +			break;
> > +
> > +		/*
> > +		 * The guest must retry the operation for the pages in the
> > +		 * region starting at the GPA specified in R11. Make sure R11
> > +		 * contains a sane value.
> > +		 */
> > +		if ((r11 & ~cc_mkdec(0)) < (start & ~cc_mkdec(0)) ||
> > +		    (r11 & ~cc_mkdec(0)) >=3D (end  & ~cc_mkdec(0)))
> > +			return false;
>=20
> Emm. All of them suppose to have shared bit set, why not compare directly
> without cc_mkdec() dance?

The above code is unnecessary and will be removed.

So, I'll use the below in v2:

/*
 * Notify the VMM about page mapping conversion. More info about ABI
 * can be found in TDX Guest-Host-Communication Interface (GHCI),
 * section "TDG.VP.VMCALL<MapGPA>"
 */
static bool tdx_map_gpa(phys_addr_t start, phys_addr_t end, bool enc)
{
        int max_retry_cnt =3D 1000, retry_cnt =3D 0;
        struct tdx_hypercall_args args;
        u64 map_fail_paddr, ret;

        while (1) {
                args.r10 =3D TDX_HYPERCALL_STANDARD;
                args.r11 =3D TDVMCALL_MAP_GPA;
                args.r12 =3D start;
                args.r13 =3D end - start;
                args.r14 =3D 0;
                args.r15 =3D 0;

                ret =3D __tdx_hypercall(&args, TDX_HCALL_HAS_OUTPUT);
                if (!ret)
                        break;

                if (ret !=3D TDVMCALL_STATUS_RETRY)
                        break;
                /*
                 * The guest must retry the operation for the pages in the
                 * region starting at the GPA specified in R11. Make sure R=
11
                 * contains a sane value.
                 */
                map_fail_paddr =3D args.r11 ;
                if (map_fail_paddr < start || map_fail_paddr >=3D end)
                        return false;

                if (map_fail_paddr =3D=3D start) {
                        retry_cnt++;
                        if (retry_cnt > max_retry_cnt)
                                return false;
                } else {
                        retry_cnt =3D 0;;
                        start =3D map_fail_paddr;
                }
        }

        return !ret;
}