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Date: Fri, 23 Feb 2024 11:39:07 -0800
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Subject: Re: [PATCHv7 10/16] x86/tdx: Convert shared memory back to private on
 kexec
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To: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
 Thomas Gleixner <tglx@linutronix.de>, Ingo Molnar <mingo@redhat.com>,
 Borislav Petkov <bp@alien8.de>, Dave Hansen <dave.hansen@linux.intel.com>,
 x86@kernel.org
Cc: "Rafael J. Wysocki" <rafael@kernel.org>,
 Peter Zijlstra <peterz@infradead.org>,
 Adrian Hunter <adrian.hunter@intel.com>,
 Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@linux.intel.com>,
 Elena Reshetova <elena.reshetova@intel.com>,
 Jun Nakajima <jun.nakajima@intel.com>,
 Rick Edgecombe <rick.p.edgecombe@intel.com>,
 Tom Lendacky <thomas.lendacky@amd.com>, "Kalra, Ashish"
 <ashish.kalra@amd.com>, Sean Christopherson <seanjc@google.com>,
 "Huang, Kai" <kai.huang@intel.com>, Baoquan He <bhe@redhat.com>,
 kexec@lists.infradead.org, linux-coco@lists.linux.dev,
 linux-kernel@vger.kernel.org
References: <20240212104448.2589568-1-kirill.shutemov@linux.intel.com>
 <20240212104448.2589568-11-kirill.shutemov@linux.intel.com>
From: Dave Hansen <dave.hansen@intel.com>
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In-Reply-To: <20240212104448.2589568-11-kirill.shutemov@linux.intel.com>
Content-Type: text/plain; charset=UTF-8
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On 2/12/24 02:44, Kirill A. Shutemov wrote:
> +static void tdx_kexec_stop_conversion(bool crash)
> +{
> +	/* Stop new private<->shared conversions */
> +	conversion_allowed = false;
> +
> +	/*
> +	 * Make sure conversion_allowed is cleared before checking
> +	 * conversions_in_progress.
> +	 */
> +	barrier();
> +
> +	/*
> +	 * Crash kernel reaches here with interrupts disabled: can't wait for
> +	 * conversions to finish.
> +	 *
> +	 * If race happened, just report and proceed.
> +	 */
> +	if (!crash) {
> +		unsigned long timeout;
> +
> +		/*
> +		 * Wait for in-flight conversions to complete.
> +		 *
> +		 * Do not wait more than 30 seconds.
> +		 */
> +		timeout = 30 * USEC_PER_SEC;
> +		while (atomic_read(&conversions_in_progress) && timeout--)
> +			udelay(1);
> +	}
> +
> +	if (atomic_read(&conversions_in_progress))
> +		pr_warn("Failed to finish shared<->private conversions\n");
> +}

I'd really prefer we find a way to do this with actual locks, especially
'conversion_allowed'.

This is _awfully_ close to being able to be handled by a rwsem where the
readers are the converters and tdx_kexec_stop_conversion() takes a write.