From: Paolo Bonzini <pbonzini@redhat.com>
To: Stefan Hajnoczi <stefanha@redhat.com>,
Richard Henderson <richard.henderson@linaro.org>
Cc: Florian Weimer <fweimer@redhat.com>,
qemu-devel@nongnu.org, Serge Guelton <sguelton@redhat.com>
Subject: Re: Portable inline asm to get address of TLS variable
Date: Wed, 16 Feb 2022 23:28:34 +0100 [thread overview]
Message-ID: <a35b3b64-2e6b-2a43-9f2b-ab3ba322bbea@redhat.com> (raw)
In-Reply-To: <Yg04Y05ccrbFVmG/@stefanha-x1.localdomain>
On 2/16/22 18:46, Stefan Hajnoczi wrote:
> However, I wonder if the compiler might reuse a register that already
> contains the address. Then we'd have the coroutine problem again when
> qemu_coroutine_yield() is called between the earlier address calculation
> and the asm volatile statement.
Yes, the compiler should be able to reuse the register. volatile only
says that the contents of the "asm" cannot be subject to e.g. loop
optimizations:
for (i = 0; i < 10; i++) {
asm("# assembly": "=r"(k) : "0"(10));
j += k;
}
will likely execute the asm once, while "asm volatile" (or an asm
without inputs, which is always volatile) will execute it ten times.
However, the input of the assembly can be evaluated only once either
way. For example, in the case above you might have "movl $10, %edx"
outside the loop even with asm volatile.
One way to fix it for modules could be to define a (global, non-TLS)
variable in QEMU with the %fs-based offset of the relevant thread-local
variable, and initialize it before modules are loaded.
Paolo
prev parent reply other threads:[~2022-02-16 22:29 UTC|newest]
Thread overview: 19+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-02-16 17:46 Portable inline asm to get address of TLS variable Stefan Hajnoczi
2022-02-16 18:13 ` Florian Weimer
2022-02-16 20:28 ` Stefan Hajnoczi
2022-02-16 20:33 ` Stefan Hajnoczi
2022-02-16 20:46 ` Florian Weimer
2022-02-17 9:30 ` Stefan Hajnoczi
2022-02-16 20:40 ` Florian Weimer
2022-02-17 9:28 ` Stefan Hajnoczi
2022-02-17 11:40 ` Paolo Bonzini
2022-02-17 15:02 ` Serge Guelton
2022-02-17 15:11 ` Stefan Hajnoczi
2022-02-17 15:51 ` Paolo Bonzini
2022-02-17 14:59 ` Serge Guelton
2022-03-01 11:54 ` Florian Weimer
2022-03-01 13:39 ` Stefan Hajnoczi
2022-04-19 11:32 ` Florian Weimer
2022-04-19 18:38 ` Thomas Rodgers
2022-04-20 14:12 ` Stefan Hajnoczi
2022-02-16 22:28 ` Paolo Bonzini [this message]
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