Re: [PATCH v14 08/10] Adding info [tb-list|tb] commands to HMP (WIP)

[Peter Maydell]

On Fri, 9 Jun 2023 at 15:32, Wu, Fei <fei2.wu@intel.com> wrote:
>
> On 6/8/2023 5:23 PM, Peter Maydell wrote:
> > On Thu, 8 Jun 2023 at 08:44, Wu, Fei <fei2.wu@intel.com> wrote:
> >> Is there any existing function to convert ram_addr_t to guest pa?
> >
> > Such a function would not be well-defined, because a block of RAM
> > as specified by a ram_addr_t could be present at (aliased to) multiple
> > guest physical addresses, or even currently not mapped to any guest
> > physical address at all. And it could be present at different physical
> > addresses for different vCPUs.
> >
> Thank you, Peter. What's the scenario of the last different physical
> addresses for different vCPUs?

You can have a board with two different CPUs, where one of them
has a RAM MemoryRegion that it puts at address A in its address
space, and the other puts it at address B. This is most likely
with 'heterogenous' configurations where you have an application
processor A and a board-support processor B. (I don't know if
we actually have any board models like that currently, but it's
logically possible.)

> For this specific case, I found I don't have to convert ram_addr_t to
> gpa, the real requirement is converting ram_addr_t to hva, this one
> seems well defined using qemu_map_ram_ptr(0, ram_addr) ?

That does work for ram_addr_t to hva, but
but note the warning on the comment above that function:
you need to be in an RCU critical section to avoid some other
thread coming along and de-allocating the RAM under your feet.

(Also it's tempting to remove that support for passing in 0
as the RAMBlock, because every other use in the codebase seems
to pass in a RAMBlock pointer.)

thanks
-- PMM