use the stfle instruction rather than the auxvec

the `vector_packed_decimal` facility does not have a bit in the auxvec from what I can tell, also the code is just much shorter, so a win all around

Author: folkertdev

crates/std_detect/src/detect/os/linux/s390x.rs

@@ -1,82 +1,37 @@
 //! Run-time feature detection for s390x on Linux.
 
-use super::auxvec;
-use crate::detect::{Feature, bit, cache};
+use crate::detect::{Feature, cache};
 
 /// Try to read the features from the auxiliary vector
 pub(crate) fn detect_features() -> cache::Initializer {
-    if let Ok(auxv) = auxvec::auxv() {
-        let hwcap: AtHwcap = auxv.into();
-        return hwcap.cache();
-    }
-
-    cache::Initializer::default()
-}
-
-/// These values are part of the platform-specific [asm/elf.h][kernel], and are a selection of the
-/// fields found in the [Facility Indications].
-///
-/// [Facility Indications]: https://www.ibm.com/support/pages/sites/default/files/2021-05/SA22-7871-10.pdf#page=63
-/// [kernel]: https://github.com/torvalds/linux/blob/b62cef9a5c673f1b8083159f5dc03c1c5daced2f/arch/s390/include/asm/elf.h#L129
-#[derive(Debug, Default, PartialEq)]
-struct AtHwcap {
-    esan3: bool,
-    zarch: bool,
-    stfle: bool,
-    msa: bool,
-    ldisp: bool,
-    eimm: bool,
-    dfp: bool,
-    hpage: bool,
-    etf3eh: bool,
-    high_gprs: bool,
-    te: bool,
-    vxrs: bool,
-    vxrs_bcd: bool,
-    vxrs_ext: bool,
-    gs: bool,
-    vxrs_ext2: bool,
-    vxrs_pde: bool,
-    sort: bool,
-    dflt: bool,
-    vxrs_pde2: bool,
-    nnpa: bool,
-    pci_mio: bool,
-    sie: bool,
+    ExtendedFacilityList::new().cache()
 }
 
-impl From<auxvec::AuxVec> for AtHwcap {
-    /// Reads AtHwcap from the auxiliary vector.
-    fn from(auxv: auxvec::AuxVec) -> Self {
-        AtHwcap {
-            esan3: bit::test(auxv.hwcap, 0),
-            zarch: bit::test(auxv.hwcap, 1),
-            stfle: bit::test(auxv.hwcap, 2),
-            msa: bit::test(auxv.hwcap, 3),
-            ldisp: bit::test(auxv.hwcap, 4),
-            eimm: bit::test(auxv.hwcap, 5),
-            dfp: bit::test(auxv.hwcap, 6),
-            hpage: bit::test(auxv.hwcap, 7),
-            etf3eh: bit::test(auxv.hwcap, 8),
-            high_gprs: bit::test(auxv.hwcap, 9),
-            te: bit::test(auxv.hwcap, 10),
-            vxrs: bit::test(auxv.hwcap, 11),
-            vxrs_bcd: bit::test(auxv.hwcap, 12),
-            vxrs_ext: bit::test(auxv.hwcap, 13),
-            gs: bit::test(auxv.hwcap, 14),
-            vxrs_ext2: bit::test(auxv.hwcap, 15),
-            vxrs_pde: bit::test(auxv.hwcap, 16),
-            sort: bit::test(auxv.hwcap, 17),
-            dflt: bit::test(auxv.hwcap, 18),
-            vxrs_pde2: bit::test(auxv.hwcap, 19),
-            nnpa: bit::test(auxv.hwcap, 20),
-            pci_mio: bit::test(auxv.hwcap, 21),
-            sie: bit::test(auxv.hwcap, 22),
+struct ExtendedFacilityList([u64; 4]);
+
+impl ExtendedFacilityList {
+    fn new() -> Self {
+        let mut result: [u64; 4] = [0; 4];
+        // SAFETY: the s390x-unknown-linux-{gnu, musl} targets assume a minimum architecture level of z10,
+        // which guarantees support for the the stfle instruction.
+        unsafe {
+            core::arch::asm!(
+                "lgr %r0, {0}",
+                // equivalently ".insn s, 0xb2b00000, 0({1})",
+                "stfle 0({1})",
+                in(reg) result.len() as u64 - 1,
+                in(reg_addr) result.as_mut_ptr() ,
+                options(nostack, preserves_flags )
+            );
         }
+        Self(result)
+    }
+
+    const fn get(&self, n: usize) -> bool {
+        // of course they number bits from the left...
+        self.0[n / 64] & (1 << (63 - (n % 64))) != 0
     }
-}
 
-impl AtHwcap {
     /// Initializes the cache from the feature bits.
     fn cache(self) -> cache::Initializer {
         let mut value = cache::Initializer::default();
@@ -87,45 +42,26 @@ impl AtHwcap {
                 }
             };
 
-            // vector and related
-
-            // bit 129 of the extended facility list
-            enable_feature(Feature::vector, self.vxrs);
-
-            // bit 135 of the extended facility list
-            enable_feature(Feature::vector_enhancements_1, self.vxrs_ext);
-
-            // bit 148 of the extended facility list
-            enable_feature(Feature::vector_enhancements_2, self.vxrs_ext2);
+            // facility indications are taken from the IBM docs
+            // https://www.ibm.com/support/pages/sites/default/files/2021-05/SA22-7871-10.pdf#page=63
 
-            // bit 134 of the extended facility list
-            enable_feature(Feature::vector_packed_decimal, self.dfp); // TODO I think this is wrong
-
-            // bit 152 of the extended facility list
-            enable_feature(Feature::vector_packed_decimal_enhancement, self.vxrs_pde);
-
-            // bit 192 of the extended facility list
-            enable_feature(Feature::vector_packed_decimal_enhancement, self.vxrs_pde2);
-
-            // bit 165 of the extended facility list
-            enable_feature(Feature::nnp_assist, self.nnpa);
-
-            // others
-
-            // bit 45 of the extended facility list
-            enable_feature(Feature::high_word, self.high_gprs);
-
-            // bit 73 of the extended facility list
-            enable_feature(Feature::transactional_execution, self.te);
-
-            // bit 133 of the extended facility list
-            enable_feature(Feature::guarded_storage, self.gs);
-
-            // bit 150 of the extended facility list
-            enable_feature(Feature::enhanced_sort, self.sort);
-
-            // bit 151 of the extended facility list
-            enable_feature(Feature::deflate_conversion, self.dflt);
+            // vector and related
+            enable_feature(Feature::vector, self.get(129));
+            enable_feature(Feature::vector_enhancements_1, self.get(135));
+            enable_feature(Feature::vector_enhancements_2, self.get(148));
+
+            enable_feature(Feature::vector_packed_decimal, self.get(134));
+            enable_feature(Feature::vector_packed_decimal_enhancement, self.get(152));
+            enable_feature(Feature::vector_packed_decimal_enhancement_2, self.get(192));
+
+            enable_feature(Feature::nnp_assist, self.get(165));
+
+            // other
+            enable_feature(Feature::high_word, self.get(45));
+            enable_feature(Feature::transactional_execution, self.get(73));
+            enable_feature(Feature::guarded_storage, self.get(133));
+            enable_feature(Feature::enhanced_sort, self.get(150));
+            enable_feature(Feature::deflate_conversion, self.get(151));
         }
         value
     }