Comment 8 for bug 1091032

from: drivers/acpi/apei/ghes.c (this is git head rather than any specific version):

        if (hest_disable) {
                pr_info(GHES_PFX "HEST is not enabled!\n");
                return -EINVAL;
        }

then drivers/acpi/apei/hest.c:

bool hest_disable;
EXPORT_SYMBOL_GPL(hest_disable);

static int __init setup_hest_disable(char *str)
{
        hest_disable = 1;
        return 0;
}

and it's documented as a flag in ./Documentation/kernel-parameters.txt :
        hest_disable [ACPI]
                        Disable Hardware Error Source Table (HEST) support;
                        corresponding firmware-first mode error processing
                        logic will be disabled.

I can't actually see anything that sets hest_disable to anything other than 1.