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Thu, 21 Aug 2025 15:56:09 -0700 (PDT) Message-ID: <8229c4f7-e1ca-4d79-9131-df5d74e464bc@citrix.com> Date: Thu, 21 Aug 2025 23:56:07 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH 20/22] x86/traps: Alter switch_stack_and_jump() for FRED mode To: Jan Beulich Cc: =?UTF-8?Q?Roger_Pau_Monn=C3=A9?= , Xen-devel References: <20250808202314.1045968-1-andrew.cooper3@citrix.com> <20250808202314.1045968-21-andrew.cooper3@citrix.com> <7b46de92-d7b6-462f-8469-0f334d9e70bb@suse.com> <17498f02-0603-489c-bbc9-2207ffef8029@citrix.com> <0fdea579-7100-43d8-b5ec-0c9d34476aea@suse.com> Content-Language: en-GB From: Andrew Cooper Autocrypt: addr=andrew.cooper3@citrix.com; keydata= xsFNBFLhNn8BEADVhE+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+cWwARAQABzSlBbmRyZXcgQ29v cGVyIDxhbmRyZXcuY29vcGVyM0BjaXRyaXguY29tPsLBegQTAQgAJAIbAwULCQgHAwUVCgkI 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 TQTBLzDKXok86M7BTQRS4TZ/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 uAJyDDKN99ziC0Wz5kcPyVD1HNf8bgaqGDzrv3TfYjwqayRFcMf7xJaL9xXedMcAEQEAAcLB XwQYAQgACQUCUuE2fwIbDAAKCRBlw/kGpdefoG4XEACD1Qf/er8EA7g23HMxYWd3FXHThrVQ HgiGdk5Yh632vjOm9L4sd/GCEACVQKjsu98e8o3ysitFlznEns5EAAXEbITrgKWXDDUWGYxd pnjj2u+GkVdsOAGk0kxczX6s+VRBhpbBI2PWnOsRJgU2n10PZ3mZD4Xu9kU2IXYmuW+e5KCA vTArRUdCrAtIa1k01sPipPPw6dfxx2e5asy21YOytzxuWFfJTGnVxZZSCyLUO83sh6OZhJkk b9rxL9wPmpN/t2IPaEKoAc0FTQZS36wAMOXkBh24PQ9gaLJvfPKpNzGD8XWR5HHF0NLIJhgg 4ZlEXQ2fVp3XrtocHqhu4UZR4koCijgB8sB7Tb0GCpwK+C4UePdFLfhKyRdSXuvY3AHJd4CP 4JzW0Bzq/WXY3XMOzUTYApGQpnUpdOmuQSfpV9MQO+/jo7r6yPbxT7CwRS5dcQPzUiuHLK9i nvjREdh84qycnx0/6dDroYhp0DFv4udxuAvt1h4wGwTPRQZerSm4xaYegEFusyhbZrI0U9tJ B8WrhBLXDiYlyJT6zOV2yZFuW47VrLsjYnHwn27hmxTC/7tvG3euCklmkn9Sl9IAKFu29RSo d5bD8kMSCYsTqtTfT6W4A3qHGvIDta3ptLYpIAOD2sY3GYq2nf3Bbzx81wZK14JdDDHUX2Rs 6+ahAA== In-Reply-To: <0fdea579-7100-43d8-b5ec-0c9d34476aea@suse.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit On 15/08/2025 10:10 am, Jan Beulich wrote: > On 14.08.2025 22:55, Andrew Cooper wrote: >> On 14/08/2025 4:35 pm, Jan Beulich wrote: >>> On 08.08.2025 22:23, Andrew Cooper wrote: >>>> FRED and IDT differ by a Supervisor Token on the base of the shstk. This >>>> means that switch_stack_and_jump() needs to discard one extra word when FRED >>>> is active. >>>> >>>> Signed-off-by: Andrew Cooper >>>> --- >>>> CC: Jan Beulich >>>> CC: Roger Pau Monné >>>> >>>> RFC. I don't like this, but it does work. >>>> >>>> This emits opt_fred logic outside of CONFIG_XEN_SHSTK. >>> opt_fred and XEN_SHSTK are orthogonal, so that's fine anyway. What I guess >>> you may mean is that you now have a shstk-related calculation outside of >>> a respective #ifdef. >> I really mean "outside of the path where shadow stacks are known to be >> active", i.e. inside the middle of SHADOW_STACK_WORK >> >>> Given the simplicity of the calculation, ... >>> >>>> But frankly, the >>>> construct is already too unweildly, and all options I can think of make it >>>> moreso. >>> ... I agree having it like this is okay. >> Yes, but it is a read of a global even when it's not used. >> >> And as a tangent, we probably want __ro_after_init_read_mostly too.  The >> read mostly is about cache locality, and is applicable even to the >> __ro_after_init section. > Not really: __read_mostly is to keep stuff rarely written apart from stuff > more frequently written (cache locality, yes). There's not going to be any > frequently written data next to a __ro_after_init item; it's all r/o post- > boot. And I don't think we care much during boot. It's not about boot, but hot variables do need grouping.  opt_fred is read on fastpaths, whereas trampoline_phys is not. > >>>> @@ -154,7 +155,6 @@ unsigned long get_stack_dump_bottom (unsigned long sp); >>>> "rdsspd %[ssp];" \ >>>> "cmp $1, %[ssp];" \ >>>> "je .L_shstk_done.%=;" /* CET not active? Skip. */ \ >>>> - "mov $%c[skstk_base], %[val];" \ >>>> "and $%c[stack_mask], %[ssp];" \ >>>> "sub %[ssp], %[val];" \ >>>> "shr $3, %[val];" \ >>> With the latter two insns here, ... >>> >>>> @@ -177,6 +177,8 @@ unsigned long get_stack_dump_bottom (unsigned long sp); >>>> >>>> #define switch_stack_and_jump(fn, instr, constr) \ >>>> ({ \ >>>> + unsigned int token_offset = \ >>>> + (PRIMARY_SHSTK_SLOT + 1) * PAGE_SIZE - (opt_fred ? 0 : 8); \ >>>> unsigned int tmp; \ >>>> BUILD_BUG_ON(!ssaj_has_attr_noreturn(fn)); \ >>>> __asm__ __volatile__ ( \ >>>> @@ -184,12 +186,11 @@ unsigned long get_stack_dump_bottom (unsigned long sp); >>>> "mov %[stk], %%rsp;" \ >>>> CHECK_FOR_LIVEPATCH_WORK \ >>>> instr "[fun]" \ >>>> - : [val] "=&r" (tmp), \ >>>> + : [val] "=r" (tmp), \ >>> ... I don't think you can legitimately drop the & from here? With it >>> retained: >>> Reviewed-by: Jan Beulich >> You chopped the bit which has an explicit input for "[val]", making the >> earlyclobber incorrect. > I was wondering whether there was a connection there, but ... > >> IIRC, one version of Clang complained. > ... that's not good. Without the early-clobber the asm() isn't quite > correct imo. If the same value appeared as another input, the compiler > may validly tie both together, assuming the register stays intact until > the very last insn (and hence even that last insn could still use the > register as an input). IOW if there's a Clang issue here, I think it > may need working around explicitly. Given that I need an alternative anyway, this becomes much easier, and shrinks to this single hunk: diff --git a/xen/arch/x86/include/asm/current.h b/xen/arch/x86/include/asm/current.h index c1eb27b1c4c2..35cc61fa88e7 100644 --- a/xen/arch/x86/include/asm/current.h +++ b/xen/arch/x86/include/asm/current.h @@ -154,7 +154,9 @@ unsigned long get_stack_dump_bottom (unsigned long sp);      "rdsspd %[ssp];"                                            \      "cmp $1, %[ssp];"                                           \      "je .L_shstk_done.%=;" /* CET not active?  Skip. */         \ -    "mov $%c[skstk_base], %[val];"                              \ +    ALTERNATIVE("mov $%c[skstk_base], %[val];",                 \ +                "mov $%c[skstk_base] + 8, %[val];",             \ +                X86_FEATURE_XEN_FRED)                           \      "and $%c[stack_mask], %[ssp];"                              \      "sub %[ssp], %[val];"                                       \      "shr $3, %[val];"                                           \ ~Andrew