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SubjectRe: [PATCH] Add Documentation/kvm/msr.txt
On 05/26/2010 09:04 PM, Glauber Costa wrote:
> This patch adds a file that documents the usage of KVM-specific
> MSRs.
>
>

Looks good. A few comments:

> +
> +Custom MSR list
> +--------
> +
> +The current supported Custom MSR list is:
> +
> +MSR_KVM_WALL_CLOCK: 0x11
> +
> + data: physical address of a memory area.

Which must be in guest RAM (i.e., don't point it somewhere random and
expect the hypervisor to allocate it for you).

Must be aligned to 4 bytes (we don't enforce it though).

> +
> +MSR_KVM_SYSTEM_TIME: 0x12
> +
> + data: physical address of a memory area. This memory is expected to
> + hold a copy of the following structure:
>

In guest RAM. What are the alignment restrictions? I don't think we
can restrict to less than 4 bytes without breaking guests retroactively.

> +
> + struct pvclock_vcpu_time_info {
> + u32 version;
> + u32 pad0;
> + u64 tsc_timestamp;
> + u64 system_time;
> + u32 tsc_to_system_mul;
> + s8 tsc_shift;
> + u8 flags;
> + u8 pad[2];
> + } __attribute__((__packed__)); /* 32 bytes */
> +
> + whose data will be filled in by the hypervisor periodically. Only one
> + write, or registration, is needed for each VCPU. The interval between
> + updates of this structure is arbitrary, and implementation-dependent.
> +
> + version: guest has to check version before and after grabbing
> + time information, and check that they are both equal and even.
> + An odd version indicates an in-progress update.
> +
> + tsc_timestamp: the tsc value at the current VCPU, at the time
> + of the update of this structure. Guests can subtract this value
> + from current tsc to derive a notion of elapsed time since the
> + structure update.
> +
> + system_time: the current system time at the time this structure
> + was last updated. Unit is nanoseconds.
>

What is the baseline for system_time? Guest boot?

> +
> + tsc_to_system_mul: a function of the tsc frequency. One has
> + to multiply any tsc-related quantity by this value to get
> + a value in nanoseconds, besides dividing by 2^tsc_shift
> +
> + tsc_shift: cycle to nanosecond divider, as a power of two, to
> + allow for shift rights. One has to shift right any tsc-related
> + quantity by this value to get a value in nanoseconds, besides
> + multiplying by tsc_to_system_mul.
>

Would be good to write down the formula for calculating time here.

> +
> + flags: bits in this field indicate extended capabilities
> + coordinated between the guest and the hypervisor. Availability
> + of specific flags has to be checked in 0x40000001 cpuid leaf.
> + Refer to cpuid.txt for details.
>

Need to document bit 0 here for completeness. cpuid.txt documents how
to discover it, here we document how to use it.

> +
> + Availability of this MSR must be checked via bit 0 in 0x4000001 cpuid
> + leaf prior to usage.
> +
> + This MSR falls outside the reserved KVM range, and may be removed in the
> + future. Its usage is deprecated.
> +
> +MSR_KVM_WALL_CLOCK_NEW: 0x4b564d00
> +
> + data and functioning: same as MSR_KVM_WALL_CLOCK. Use this instead.
> +
> + Availability of this MSR must be checked via bit 3 in 0x4000001 cpuid
> + leaf prior to usage.
> +
> +MSR_KVM_SYSTEM_TIME_NEW: 0x4b564d01
> +
> + data and functioning: same as MSR_KVM_SYSTEM_TIME. Use this instead.
> +
> + Availability of this MSR must be checked via bit 3 in 0x4000001 cpuid
> + leaf prior to usage.
> +
>

Please detail the suggested algorithm for using the msrs (new then old).



--
error compiling committee.c: too many arguments to function



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