[crac] RFR: 8373027: [CRaC] [CRIU] x86: FPU xsave area present, but host cpu doesn't support it

[Jan Kratochvil]

On Wed, 3 Dec 2025 15:08:08 GMT, Jan Kratochvil <jkratochvil at openjdk.org> wrote:

> In some cases (such as AWS `t3.nano` -> `m1.small`) CRIU refuses to restore:
> 
> x86: FPU xsave area present, but host cpu doesn't support it
> 
> 
> I think (based on information of @rvansa `xsave` is not needed for the safepoint of JDK checkpoint/restore.

The JVM process may run native threads which are fully out of control JVM `_features`. So the whole CPUFeatures can always fail for CRaC-unaware application. But otherwise I tried it with FPU Java app and it works for me:

import jdk.incubator.vector.DoubleVector;
import jdk.incubator.vector.VectorSpecies;
import jdk.incubator.vector.VectorOperators;
import java.util.Random;
public class CPUFeaturesAWS {
    static final int SIZE = 16_000_000;
    static final double[] array = new double[SIZE];
    public static void main(String[] args) {
        Random rnd = new Random(42);
        for (int i = 0; i < SIZE; i++) array[i] = rnd.nextDouble();
        VectorSpecies<Double> SPECIES = DoubleVector.SPECIES_512; 
        while (true) {
            double sum = 0.0;
            int i = 0;
            for (; i + SPECIES.length() <= SIZE; i += SPECIES.length()) {
                var vec = DoubleVector.fromArray(SPECIES, array, i);
                sum += vec.reduceLanes(VectorOperators.ADD); 
            }
            for (; i < SIZE; i++) sum += array[i]; 
            double expected = expectedSum();
            if (Math.abs(sum - expected) > 1e-6) {
                System.err.println("Mismatch! sum=" + sum + " expected=" + expected);
                System.exit(1);
            }
System.out.println("CPUFeaturesCheck " + 42);
        }
    }
    private static double expectedSum() {
        double s = 0.0;
        for (double v : array) s += v;
        return s;
    }
}

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PR Comment: https://git.openjdk.org/crac/pull/279#issuecomment-3656836579