linux-kernel - Re: [PATCH 1/3] include files for kvmclock

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Date:	Fri, 09 Nov 2007 09:37:30 +0100
From:	Gerd Hoffmann <kraxel@...hat.com>
To:	Glauber de Oliveira Costa <gcosta@...hat.com>
CC:	linux-kernel@...r.kernel.org, jeremy@...p.org, avi@...ranet.com,
	aliguori@...ibm.com, kvm-devel@...ts.sourceforge.net,
	hollisb@...ibm.com
Subject: Re: [PATCH 1/3] include files for kvmclock

> +/*
> + * Guest has page alignment and padding requirements. At the host, it will
> + * only lead to wasted space at the vcpu struct. For this reason, the struct
> + * is not anonymous
> + */
> +union kvm_hv_clock {
> +	struct kvm_hv_clock_s {
> +		u64 tsc_mult;
> +		u64 now_ns;
> +		/* That's the wall clock, not the water closet */
> +		u64 wc_sec;
> +		u64 last_tsc;
> +		/* At first, we could use the tsc value as a marker, but Jeremy
> +		 * well noted that it will cause us locking problems in 32-bit
> +		 * sys, so we have a special version field */
> +		u32 version;
> +	} fields;
> +	char page_align[PAGE_SIZE];
> +};

What is the point in using a whole page per vcpu?  You probably don't
want struct kvm_hv_clock_s cross a page border.  Is that the only reason
or are there other constrains too?

As the kvm clock looks quite simliar to what xen does, how about making
the structs binary-compatible or simply reuse the xen version (struct
vcpu_time_info in xen/interface/xen.h)?

cheers,
  Gerd
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