Re: [PATCH v3 04/34] KVM: x86: hyper-v: Handle HVCALL_FLUSH_VIRTUAL_ADDRESS_LIST{,EX} calls gently

Sean Christopherson [off-list ref] writes:

On Wed, May 18, 2022, Vitaly Kuznetsov wrote:

quoted

Maxim Levitsky [off-list ref] writes:

quoted

Or if using kfifo, then it can contain plain u64 items, which is even more natural.

In the next version I switch to fifo and get rid of 'flush_all' entries
but instead of a boolean I use a 'magic' value of '-1' in GVA. This way
we don't need to synchronize with the reader and add any special
handling for the flag.

Isn't -1 theoretically possible?  Or is wrapping not allowed?  E.g. requesting a
flush for address=0xfffffffffffff000, count = 0xfff will yield -1 and doesn't
create any illegal addresses in the process.

Such an error would just lead to KVM flushing the whole guest address
space instead of flushing 4096 pages starting with 0xfffffffffffff000
but over-flushing is always architecturally correct, isn't it?

Personally, I'm not opposed to dropping the magic and enhancing flush
entries with 'flags' again but I'd like to avoid keeping this info
somewhere aside. Also, after we switch to kfifo, we can't play with
ring indexes to somehow indicate this special case. We probably can use
'fifo is full' as such indication but this is very, very un-obvious.

-- 
Vitaly