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Re: [Qemu-devel] [PATCH v2 1/6] target/s390x: Implement STORE FACILITIES


From: Richard Henderson
Subject: Re: [Qemu-devel] [PATCH v2 1/6] target/s390x: Implement STORE FACILITIES LIST EXTENDED
Date: Tue, 9 May 2017 09:28:13 -0700
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.1.0

On 05/09/2017 08:17 AM, Aurelien Jarno wrote:
Ok, thanks for the detailed explanations. Then I guess you should fold
the following patch to correctly set the zArch active bit as done in
s390_fill_feat_block:

--- a/target/s390x/misc_helper.c
+++ b/target/s390x/misc_helper.c
@@ -693,6 +693,11 @@ static unsigned do_stfle(CPUS390XState *env, uint64_t 
words[MAX_STFL_WORDS])
memset(words, 0, sizeof(uint64_t) * MAX_STFL_WORDS); + if (test_bit(S390_FEAT_ZARCH, features)) {
+        /* z/Architecture is always active if around */
+        words[0] |= 1ull << 61;
+    }

Oops, yes, good catch.

Which means that we cannot mask *anything* from TCG, including the useless
stuff like hexadecimal floating point, and still have the kernel boot.  So
in practice a feature mask for TCG will be not only useless but actively
harmful.

That's true if you want to boot an optimized kernel.

Well, yes. But the last time I was really looking at this (2 years ago?) Fedora was built to use z9-109. I guess maybe Debian is the only distribution that doesn't force some later cpu?

As soon as you try to
use a newer CPU like the z990, the DAT-enhancement facility bit is set
in STFL, and the kernel try to use the idte instruction which is not
emulated by QEMU.

Good to know.  Thanks,


r~



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