Re: [PATCH] virt: tdx-guest: Poll Quote status with exponential backoff

[Kiryl Shutsemau <kas@kernel.org>]

On Wed, Oct 21, 2026 at 05:16:32PM +0000, Moe Mahhouk via B4 Relay wrote:
> The latency was found while measuring TD boot and attestation times.
> The code and this changelog were written with an AI coding assistant,
> Claude (Claude Code, model Claude Opus 5.5), which also ran the tests.
> Tested on mainline a804c5742 in a TDX guest (KVM host v7.0, QEMU 11.1.2,
> TDX module 1.5.34, Intel QGS) with 40 Quote requests per kernel; builds
> with W=1 and passes checkpatch.pl.

Does anybody find this kind of dump useful? Maybe move after --- ?

> @@ -271,18 +273,25 @@ static struct tdx_quote_buf *alloc_quote_buf(void)
>   */
>  static int wait_for_quote_completion(struct tdx_quote_buf *buf, u32 timeout)
>  {
> -	int i = 0;
> +	unsigned long deadline = jiffies + timeout * HZ;
> +	unsigned int sleep_us = 100;
>  
>  	/*
> -	 * Quote requests usually take a few seconds to complete, so waking up
> -	 * once per second to recheck the status is fine for this use case.
> +	 * A local Quote Generation Service usually completes the request
> +	 * within a few milliseconds. Poll with an exponential backoff, from
> +	 * 100 us up to 100 ms, so that a fast Quote is noticed promptly while
> +	 * a slow one costs at most ten wakeups per second.
>  	 */
> -	while (buf->status == GET_QUOTE_IN_FLIGHT && i++ < timeout) {
> -		if (msleep_interruptible(MSEC_PER_SEC))
> +	while (READ_ONCE(buf->status) == GET_QUOTE_IN_FLIGHT) {
> +		if (time_after(jiffies, deadline))
> +			return -ETIMEDOUT;
> +		if (signal_pending(current))
>  			return -EINTR;
> +		usleep_range(sleep_us, 2 * sleep_us);

usleep_range() in uninterruptible.

Why not use the same msleep_interruptible(), but with shorten interval?

        unsigned long deadline = jiffies + secs_to_jiffies(timeout);
        unsigned int sleep_ms = 1;

        while (READ_ONCE(buf->status) == GET_QUOTE_IN_FLIGHT) {
                if (time_after(jiffies, deadline))
                        return -ETIMEDOUT;
                if (msleep_interruptible(sleep_ms))
                        return -EINTR;
                sleep_ms = min(2 * sleep_ms, 100);
        }

        return 0;


> +		sleep_us = min(2 * sleep_us, 100 * USEC_PER_MSEC);
>  	}
>  
> -	return (i == timeout) ? -ETIMEDOUT : 0;
> +	return 0;

Hm. The current code is broken: on a real timeout i ends up at
timeout + 1

-- 
  Kiryl Shutsemau / Kirill A. Shutemov