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Re: [Xen-devel] [PATCH 2/2] xen: make xen_qlock_wait() nestable
- To: Thomas Gleixner <tglx@xxxxxxxxxxxxx>, David Woodhouse <dwmw2@xxxxxxxxxxxxx>
- From: Juergen Gross <jgross@xxxxxxxx>
- Date: Wed, 10 Oct 2018 15:38:11 +0200
- Autocrypt: addr=jgross@xxxxxxxx; prefer-encrypt=mutual; keydata= xsBNBFOMcBYBCACgGjqjoGvbEouQZw/ToiBg9W98AlM2QHV+iNHsEs7kxWhKMjrioyspZKOB ycWxw3ie3j9uvg9EOB3aN4xiTv4qbnGiTr3oJhkB1gsb6ToJQZ8uxGq2kaV2KL9650I1SJve dYm8Of8Zd621lSmoKOwlNClALZNew72NjJLEzTalU1OdT7/i1TXkH09XSSI8mEQ/ouNcMvIJ NwQpd369y9bfIhWUiVXEK7MlRgUG6MvIj6Y3Am/BBLUVbDa4+gmzDC9ezlZkTZG2t14zWPvx XP3FAp2pkW0xqG7/377qptDmrk42GlSKN4z76ELnLxussxc7I2hx18NUcbP8+uty4bMxABEB AAHNHkp1ZXJnZW4gR3Jvc3MgPGpncm9zc0BzdXNlLmRlPsLAeQQTAQIAIwUCU4xw6wIbAwcL CQgHAwIBBhUIAgkKCwQWAgMBAh4BAheAAAoJELDendYovxMvi4UH/Ri+OXlObzqMANruTd4N zmVBAZgx1VW6jLc8JZjQuJPSsd/a+bNr3BZeLV6lu4Pf1Yl2Log129EX1KWYiFFvPbIiq5M5 kOXTO8Eas4CaScCvAZ9jCMQCgK3pFqYgirwTgfwnPtxFxO/F3ZcS8jovza5khkSKL9JGq8Nk czDTruQ/oy0WUHdUr9uwEfiD9yPFOGqp4S6cISuzBMvaAiC5YGdUGXuPZKXLpnGSjkZswUzY d9BVSitRL5ldsQCg6GhDoEAeIhUC4SQnT9SOWkoDOSFRXZ+7+WIBGLiWMd+yKDdRG5RyP/8f 3tgGiB6cyuYfPDRGsELGjUaTUq3H2xZgIPfOwE0EU4xwFgEIAMsx+gDjgzAY4H1hPVXgoLK8 B93sTQFN9oC6tsb46VpxyLPfJ3T1A6Z6MVkLoCejKTJ3K9MUsBZhxIJ0hIyvzwI6aYJsnOew cCiCN7FeKJ/oA1RSUemPGUcIJwQuZlTOiY0OcQ5PFkV5YxMUX1F/aTYXROXgTmSaw0aC1Jpo w7Ss1mg4SIP/tR88/d1+HwkJDVW1RSxC1PWzGizwRv8eauImGdpNnseneO2BNWRXTJumAWDD pYxpGSsGHXuZXTPZqOOZpsHtInFyi5KRHSFyk2Xigzvh3b9WqhbgHHHE4PUVw0I5sIQt8hJq 5nH5dPqz4ITtCL9zjiJsExHuHKN3NZsAEQEAAcLAXwQYAQIACQUCU4xwFgIbDAAKCRCw3p3W KL8TL0P4B/9YWver5uD/y/m0KScK2f3Z3mXJhME23vGBbMNlfwbr+meDMrJZ950CuWWnQ+d+ Ahe0w1X7e3wuLVODzjcReQ/v7b4JD3wwHxe+88tgB9byc0NXzlPJWBaWV01yB2/uefVKryAf AHYEd0gCRhx7eESgNBe3+YqWAQawunMlycsqKa09dBDL1PFRosF708ic9346GLHRc6Vj5SRA UTHnQqLetIOXZm3a2eQ1gpQK9MmruO86Vo93p39bS1mqnLLspVrL4rhoyhsOyh0Hd28QCzpJ wKeHTd0MAWAirmewHXWPco8p1Wg+V+5xfZzuQY0f4tQxvOpXpt4gQ1817GQ5/Ed/wsDtBBgB CAAgFiEEhRJncuj2BJSl0Jf3sN6d1ii/Ey8FAlrd8NACGwIAgQkQsN6d1ii/Ey92IAQZFggA HRYhBFMtsHpB9jjzHji4HoBcYbtP2GO+BQJa3fDQAAoJEIBcYbtP2GO+TYsA/30H/0V6cr/W V+J/FCayg6uNtm3MJLo4rE+o4sdpjjsGAQCooqffpgA+luTT13YZNV62hAnCLKXH9n3+ZAgJ RtAyDWk1B/0SMDVs1wxufMkKC3Q/1D3BYIvBlrTVKdBYXPxngcRoqV2J77lscEvkLNUGsu/z W2pf7+P3mWWlrPMJdlbax00vevyBeqtqNKjHstHatgMZ2W0CFC4hJ3YEetuRBURYPiGzuJXU pAd7a7BdsqWC4o+GTm5tnGrCyD+4gfDSpkOT53S/GNO07YkPkm/8J4OBoFfgSaCnQ1izwgJQ jIpcG2fPCI2/hxf2oqXPYbKr1v4Z1wthmoyUgGN0LPTIm+B5vdY82wI5qe9uN6UOGyTH2B3p hRQUWqCwu2sqkI3LLbTdrnyDZaixT2T0f4tyF5Lfs+Ha8xVMhIyzNb1byDI5FKCb
- Cc: Waiman.Long@xxxxxx, peterz@xxxxxxxxxxxxx, x86@xxxxxxxxxx, linux-kernel@xxxxxxxxxxxxxxx, stable@xxxxxxxxxxxxxxx, mingo@xxxxxxxxxx, bp@xxxxxxxxx, hpa@xxxxxxxxx, xen-devel@xxxxxxxxxxxxxxxxxxxx, boris.ostrovsky@xxxxxxxxxx
- Delivery-date: Wed, 10 Oct 2018 13:38:26 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
- Openpgp: preference=signencrypt
On 10/10/2018 14:47, Thomas Gleixner wrote:
> On Wed, 10 Oct 2018, David Woodhouse wrote:
>> On Wed, 2018-10-10 at 14:30 +0200, Thomas Gleixner wrote:
>>> On Wed, 10 Oct 2018, David Woodhouse wrote:
>>>
>>>> On Mon, 2018-10-01 at 09:16 +0200, Juergen Gross wrote:
>>>>> - /* If irq pending already clear it and return. */
>>>>> + /* Guard against reentry. */
>>>>> + local_irq_save(flags);
>>>>> +
>>>>> + /* If irq pending already clear it. */
>>>>> if (xen_test_irq_pending(irq)) {
>>>>> xen_clear_irq_pending(irq);
>>>>> - return;
>>>>> + } else if (READ_ONCE(*byte) == val) {
>>>>> + /* Block until irq becomes pending (or a spurious wakeup)
>>>>> */
>>>>> + xen_poll_irq(irq);
>>>>> }
>>>>
>>>>
>>>> Does this still allow other IRQs to wake it from xen_poll_irq()?
>>>>
>>>> In the case where process-context code is spinning for a lock without
>>>> disabling interrupts, we *should* allow interrupts to occur still...
>>>> does this?
>>>
>>> Yes. Look at it like idle HLT or WFI. You have to disable interrupt before
>>> checking the condition and then the hardware or in this case the hypervisor
>>> has to bring you back when an interrupt is raised.
>>>
>>> If that would not work then the check would be racy, because the interrupt
>>> could hit and be handled after the check and before going into
>>> HLT/WFI/hypercall and then the thing is out until the next interrupt comes
>>> along, which might be never.
>>
>> Right, but in this case we're calling into the hypervisor to poll for
>> one *specific* IRQ. Everything you say is true for that specific IRQ.
>>
>> My question is what happens to *other* IRQs. We want them, but are they
>> masked? I'm staring at the Xen do_poll() code and haven't quite worked
>> that out...
>
> Ah, sorry. That of course has to come back like HLT/WFI for any interrupt,
> but I have no idea what the Xen HV is doing there.
The Xen HV is doing it right. It is blocking the vcpu in do_poll() and
any interrupt will unblock it.
Juergen
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