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Re: [Xen-devel] [PATCH] x86/pv: Clean up cr3 handling in arch_set_info_guest()


  • To: Roger Pau Monné <roger.pau@xxxxxxxxxx>
  • From: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
  • Date: Fri, 21 Dec 2018 17:05:53 +0000
  • Autocrypt: addr=andrew.cooper3@xxxxxxxxxx; prefer-encrypt=mutual; keydata= mQINBFLhNn8BEADVhE+Hb8i0GV6mihnnr/uiQQdPF8kUoFzCOPXkf7jQ5sLYeJa0cQi6Penp VtiFYznTairnVsN5J+ujSTIb+OlMSJUWV4opS7WVNnxHbFTPYZVQ3erv7NKc2iVizCRZ2Kxn srM1oPXWRic8BIAdYOKOloF2300SL/bIpeD+x7h3w9B/qez7nOin5NzkxgFoaUeIal12pXSR Q354FKFoy6Vh96gc4VRqte3jw8mPuJQpfws+Pb+swvSf/i1q1+1I4jsRQQh2m6OTADHIqg2E ofTYAEh7R5HfPx0EXoEDMdRjOeKn8+vvkAwhviWXTHlG3R1QkbE5M/oywnZ83udJmi+lxjJ5 YhQ5IzomvJ16H0Bq+TLyVLO/VRksp1VR9HxCzItLNCS8PdpYYz5TC204ViycobYU65WMpzWe LFAGn8jSS25XIpqv0Y9k87dLbctKKA14Ifw2kq5OIVu2FuX+3i446JOa2vpCI9GcjCzi3oHV e00bzYiHMIl0FICrNJU0Kjho8pdo0m2uxkn6SYEpogAy9pnatUlO+erL4LqFUO7GXSdBRbw5 gNt25XTLdSFuZtMxkY3tq8MFss5QnjhehCVPEpE6y9ZjI4XB8ad1G4oBHVGK5LMsvg22PfMJ ISWFSHoF/B5+lHkCKWkFxZ0gZn33ju5n6/FOdEx4B8cMJt+cWwARAQABtClBbmRyZXcgQ29v cGVyIDxhbmRyZXcuY29vcGVyM0BjaXRyaXguY29tPokCOgQTAQgAJAIbAwULCQgHAwUVCgkI CwUWAgMBAAIeAQIXgAUCWKD95wIZAQAKCRBlw/kGpdefoHbdD/9AIoR3k6fKl+RFiFpyAhvO 59ttDFI7nIAnlYngev2XUR3acFElJATHSDO0ju+hqWqAb8kVijXLops0gOfqt3VPZq9cuHlh IMDquatGLzAadfFx2eQYIYT+FYuMoPZy/aTUazmJIDVxP7L383grjIkn+7tAv+qeDfE+txL4 SAm1UHNvmdfgL2/lcmL3xRh7sub3nJilM93RWX1Pe5LBSDXO45uzCGEdst6uSlzYR/MEr+5Z JQQ32JV64zwvf/aKaagSQSQMYNX9JFgfZ3TKWC1KJQbX5ssoX/5hNLqxMcZV3TN7kU8I3kjK mPec9+1nECOjjJSO/h4P0sBZyIUGfguwzhEeGf4sMCuSEM4xjCnwiBwftR17sr0spYcOpqET ZGcAmyYcNjy6CYadNCnfR40vhhWuCfNCBzWnUW0lFoo12wb0YnzoOLjvfD6OL3JjIUJNOmJy RCsJ5IA/Iz33RhSVRmROu+TztwuThClw63g7+hoyewv7BemKyuU6FTVhjjW+XUWmS/FzknSi dAG+insr0746cTPpSkGl3KAXeWDGJzve7/SBBfyznWCMGaf8E2P1oOdIZRxHgWj0zNr1+ooF /PzgLPiCI4OMUttTlEKChgbUTQ+5o0P080JojqfXwbPAyumbaYcQNiH1/xYbJdOFSiBv9rpt TQTBLzDKXok86LkCDQRS4TZ/ARAAkgqudHsp+hd82UVkvgnlqZjzz2vyrYfz7bkPtXaGb9H4 Rfo7mQsEQavEBdWWjbga6eMnDqtu+FC+qeTGYebToxEyp2lKDSoAsvt8w82tIlP/EbmRbDVn 7bhjBlfRcFjVYw8uVDPptT0TV47vpoCVkTwcyb6OltJrvg/QzV9f07DJswuda1JH3/qvYu0p vjPnYvCq4NsqY2XSdAJ02HrdYPFtNyPEntu1n1KK+gJrstjtw7KsZ4ygXYrsm/oCBiVW/OgU g/XIlGErkrxe4vQvJyVwg6YH653YTX5hLLUEL1NS4TCo47RP+wi6y+TnuAL36UtK/uFyEuPy wwrDVcC4cIFhYSfsO0BumEI65yu7a8aHbGfq2lW251UcoU48Z27ZUUZd2Dr6O/n8poQHbaTd 6bJJSjzGGHZVbRP9UQ3lkmkmc0+XCHmj5WhwNNYjgbbmML7y0fsJT5RgvefAIFfHBg7fTY/i kBEimoUsTEQz+N4hbKwo1hULfVxDJStE4sbPhjbsPCrlXf6W9CxSyQ0qmZ2bXsLQYRj2xqd1 bpA+1o1j2N4/au1R/uSiUFjewJdT/LX1EklKDcQwpk06Af/N7VZtSfEJeRV04unbsKVXWZAk uAJyDDKN99ziC0Wz5kcPyVD1HNf8bgaqGDzrv3TfYjwqayRFcMf7xJaL9xXedMcAEQEAAYkC HwQYAQgACQUCUuE2fwIbDAAKCRBlw/kGpdefoG4XEACD1Qf/er8EA7g23HMxYWd3FXHThrVQ HgiGdk5Yh632vjOm9L4sd/GCEACVQKjsu98e8o3ysitFlznEns5EAAXEbITrgKWXDDUWGYxd pnjj2u+GkVdsOAGk0kxczX6s+VRBhpbBI2PWnOsRJgU2n10PZ3mZD4Xu9kU2IXYmuW+e5KCA vTArRUdCrAtIa1k01sPipPPw6dfxx2e5asy21YOytzxuWFfJTGnVxZZSCyLUO83sh6OZhJkk b9rxL9wPmpN/t2IPaEKoAc0FTQZS36wAMOXkBh24PQ9gaLJvfPKpNzGD8XWR5HHF0NLIJhgg 4ZlEXQ2fVp3XrtocHqhu4UZR4koCijgB8sB7Tb0GCpwK+C4UePdFLfhKyRdSXuvY3AHJd4CP 4JzW0Bzq/WXY3XMOzUTYApGQpnUpdOmuQSfpV9MQO+/jo7r6yPbxT7CwRS5dcQPzUiuHLK9i nvjREdh84qycnx0/6dDroYhp0DFv4udxuAvt1h4wGwTPRQZerSm4xaYegEFusyhbZrI0U9tJ B8WrhBLXDiYlyJT6zOV2yZFuW47VrLsjYnHwn27hmxTC/7tvG3euCklmkn9Sl9IAKFu29RSo d5bD8kMSCYsTqtTfT6W4A3qHGvIDta3ptLYpIAOD2sY3GYq2nf3Bbzx81wZK14JdDDHUX2Rs 6+ahAA==
  • Cc: Julien Grall <julien.grall@xxxxxxx>, Stefano Stabellini <sstabellini@xxxxxxxxxx>, Wei Liu <wei.liu2@xxxxxxxxxx>, Jan Beulich <JBeulich@xxxxxxxx>, Xen-devel <xen-devel@xxxxxxxxxxxxx>
  • Delivery-date: Fri, 21 Dec 2018 17:06:11 +0000
  • List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
  • Openpgp: preference=signencrypt

On 21/12/2018 17:01, Roger Pau Monné wrote:
> On Fri, Dec 21, 2018 at 01:46:05PM +0000, Andrew Cooper wrote:
>> @@ -1091,12 +1091,12 @@ int arch_set_info_guest(
>>      set_bit(_VPF_in_reset, &v->pause_flags);
>>  
>>      if ( !compat )
>> -        cr3_gfn = xen_cr3_to_pfn(c.nat->ctrlreg[3]);
>> +        cr3_mfn = _mfn(xen_cr3_to_pfn(c.nat->ctrlreg[3]));
>>      else
>> -        cr3_gfn = compat_cr3_to_pfn(c.cmp->ctrlreg[3]);
>> -    cr3_page = get_page_from_gfn(d, cr3_gfn, NULL, P2M_ALLOC);
>> +        cr3_mfn = _mfn(compat_cr3_to_pfn(c.cmp->ctrlreg[3]));
>>  
>> -    if ( !cr3_page )
>> +    if ( !mfn_valid(cr3_mfn) ||
>> +         !(cr3_page = mfn_to_page(cr3_mfn), get_page(cr3_page, d)) )
> This is kind of an open-coded version of get_page_from_gfn with just
> the non-paging bits, IMO I would use get_page_from_gfn, or introduce a
> get_page_from_mfn helper?

Turns out that we already have one of those.  I'll tweak it for purpose
and use it.

>
> The more that you use the same construct below.
>
>>          rc = -EINVAL;
>>      else if ( paging_mode_refcounts(d) )
>>          /* nothing */;
>> @@ -1122,7 +1122,7 @@ int arch_set_info_guest(
>>          case 0:
>>              if ( !compat && !VM_ASSIST(d, m2p_strict) &&
>>                   !paging_mode_refcounts(d) )
>> -                fill_ro_mpt(_mfn(cr3_gfn));
>> +                fill_ro_mpt(cr3_mfn);
>>              break;
>>          default:
>>              if ( cr3_page == current->arch.old_guest_table )
>> @@ -1137,10 +1137,10 @@ int arch_set_info_guest(
>>          v->arch.guest_table = pagetable_from_page(cr3_page);
>>          if ( c.nat->ctrlreg[1] )
>>          {
>> -            cr3_gfn = xen_cr3_to_pfn(c.nat->ctrlreg[1]);
>> -            cr3_page = get_page_from_gfn(d, cr3_gfn, NULL, P2M_ALLOC);
>> +            cr3_mfn = _mfn(xen_cr3_to_pfn(c.nat->ctrlreg[1]));
> I assume this is something PV specific, but calling xen_cr3_to_pfn on
> ctrlreg[1] seems wrong at first sight. And the xen_cr3_to_pfn and
> xen_pfn_to_cr3 helpers seem quite pointless, since it's just PFN_DOWN
> or pfn_to_paddr.

It is one of the more peculiar pieces of PV magic.

For reasons best explained by whomever wrote the code, the 64bit ABI,
having separate kernel and user %cr3's, stashes the user %cr3 in ctrlreg[1].

This code is correct, but I do accept that it is very confusing to read
without knowing the PV ABI inside out.

~Andrew

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