linux-kernel - RE: [PATCH 2/6] x86/tdx: Retry TDVMCALL_MAP

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Message-ID: <SA1PR21MB133596B911C6A45142B83B52BF0C9@SA1PR21MB1335.namprd21.prod.outlook.com>
Date:   Wed, 23 Nov 2022 03:27:25 +0000
From:   Dexuan Cui <decui@...rosoft.com>
To:     "Kirill A. Shutemov" <kirill@...temov.name>
CC:     "ak@...ux.intel.com" <ak@...ux.intel.com>,
        "arnd@...db.de" <arnd@...db.de>, "bp@...en8.de" <bp@...en8.de>,
        "brijesh.singh@....com" <brijesh.singh@....com>,
        "Williams, Dan J" <dan.j.williams@...el.com>,
        "dave.hansen@...ux.intel.com" <dave.hansen@...ux.intel.com>,
        Haiyang Zhang <haiyangz@...rosoft.com>,
        "hpa@...or.com" <hpa@...or.com>,
        "jane.chu@...cle.com" <jane.chu@...cle.com>,
        "kirill.shutemov@...ux.intel.com" <kirill.shutemov@...ux.intel.com>,
        KY Srinivasan <kys@...rosoft.com>,
        "linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
        "linux-hyperv@...r.kernel.org" <linux-hyperv@...r.kernel.org>,
        "luto@...nel.org" <luto@...nel.org>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "peterz@...radead.org" <peterz@...radead.org>,
        "rostedt@...dmis.org" <rostedt@...dmis.org>,
        "sathyanarayanan.kuppuswamy@...ux.intel.com" 
        <sathyanarayanan.kuppuswamy@...ux.intel.com>,
        "seanjc@...gle.com" <seanjc@...gle.com>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "tony.luck@...el.com" <tony.luck@...el.com>,
        "wei.liu@...nel.org" <wei.liu@...nel.org>,
        "x86@...nel.org" <x86@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH 2/6] x86/tdx: Retry TDVMCALL_MAP_GPA() when needed

> From: Kirill A. Shutemov <kirill@...temov.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.
 
> >  /* 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) {
> 
> Endless? Maybe an upper limit if no progress?

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

> > +		ret = _tdx_hypercall_output_r11(TDVMCALL_MAP_GPA, start,
> > +						end - start, 0, 0, &r11);
> > +		if (!ret)
> > +			break;
> > +
> > +		if (ret != 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)) >= (end  & ~cc_mkdec(0)))
> > +			return false;
> 
> 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 = 1000, retry_cnt = 0;
        struct tdx_hypercall_args args;
        u64 map_fail_paddr, ret;

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

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

                if (ret != 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.
                 */
                map_fail_paddr = args.r11 ;
                if (map_fail_paddr < start || map_fail_paddr >= end)
                        return false;

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

        return !ret;
}