On 1/27/25 11:06 AM, Jan Beulich wrote:
On 20.01.2025 17:54, Oleksii Kurochko wrote:RISC-V doesn't have hardware feature to ask MMU to translate virtual address to physical address ( like Arm has, for example ), so software page table walking in implemented.Signed-off-by: Oleksii Kurochko<[email protected]> --- xen/arch/riscv/include/asm/mm.h | 2 ++ xen/arch/riscv/pt.c | 56 +++++++++++++++++++++++++++++++++ 2 files changed, 58 insertions(+) diff --git a/xen/arch/riscv/include/asm/mm.h b/xen/arch/riscv/include/asm/mm.h index 292aa48fc1..d46018c132 100644 --- a/xen/arch/riscv/include/asm/mm.h +++ b/xen/arch/riscv/include/asm/mm.h @@ -15,6 +15,8 @@extern vaddr_t directmap_virt_start; +paddr_t pt_walk(vaddr_t va);In the longer run, is returning just the PA really going to be sufficient? If not, perhaps say a word on the limitation in the description.
In the long run, this function's prototype looks like|paddr_t pt_walk(vaddr_t root, vaddr_t va, bool is_xen)| [1]. However, I'm not sure if it will stay that way, as I think|is_xen| could be skipped, since using|map_table()| should be sufficient (as it now considers|system_state|) and I'm not really sure if I need root argument as initial goal was to use this function for debug only purposes and I've never used it for guest page table (stage-1) walking. Anyway, yes, it is still returning a physical address, and that seems enough to me. Could you share your thoughts on what I should take into account for returning value, probably, I am missing something really useful?|[1] https://gitlab.com/xen-project/people/olkur/xen/-/blob/latest/xen/arch/riscv/mm.c#L820|
+ * initialize `ret` with "impossible" XEN_TABLE_MAP_NOMEM. + */ + int ret = XEN_TABLE_MAP_NOMEM; + unsigned int level = HYP_PT_ROOT_LEVEL; + paddr_t pa = 0;Seeing 0 as initializer here, just to double check: You do prevent MFN 0 to be handed to the page allocator, and you also don't use it "manually" anywhere?
MFN 0 could be used when removing the page:https://gitlab.com/xen-project/xen/-/blob/staging/xen/arch/riscv/pt.c?ref_type=heads#L251 <https://gitlab.com/xen-project/xen/-/blob/staging/xen/arch/riscv/pt.c?ref_type=heads#L251>. In that case, it would be better to initialize|pa| with|(paddr_t)(-1)|, as this value couldn't be mapped and is safe to use as an invalid value.
+ pte_t *table; + + DECLARE_OFFSETS(offsets, va); + + table = map_table(root); + + /* + * Find `pa` of an entry which corresponds to `va` by iterating for each + * page level and checking if the entry points to a next page table or + * to a page. + * + * Two cases are possible: + * - ret == XEN_TABLE_SUPER_PAGE means that the entry was find; + * (Despite of the name) XEN_TABLE_SUPER_PAGE covers 4k mapping too. + * - ret == XEN_TABLE_MAP_NONE means that requested `va` wasn't actually + * mapped. + */ + while ( (ret != XEN_TABLE_MAP_NONE) && (ret != XEN_TABLE_SUPER_PAGE) ) + { + /* + * This case shouldn't really occur as it will mean that for table + * level 0 a pointer to next page table has been written, but at + * level 0 it could be only a pointer to 4k page. + */ + ASSERT(level <= HYP_PT_ROOT_LEVEL); + + ret = pt_next_level(false, &table, offsets[level]); + level--; + } + + if ( ret == XEN_TABLE_MAP_NONE ) + dprintk(XENLOG_WARNING, "Is va(%#lx) really mapped?\n", va);Even if it's a dprintk(), I'd recommend against adding such.+ else if ( ret == XEN_TABLE_SUPER_PAGE ) + pa = pte_to_paddr(*(table + offsets[level + 1]));Missing "else if ( ret == XEN_TABLE_NORMAL )" (or maybe simply "else")?
If I am not missing something, we can't be here with ret == XEN_TABLE_NORMAL because we iterating in the while loop above until we don't find a leaf or until reach level = 0. If we find a leaf then XEN_TABLE_SUPER_PAGE is returned; otherwise sooner or later we should face a case when next table (in case when `level`=0 and someone put at this level a pointer to next level, what is a bug) should be allocated in pt_next_level(), but it will fail because `alloc_tbl`=false is passed to pt_next_level() and thereby ret=XEN_TABLE_MAP_NONE() will be returned. Based on your previous comment about dprintk this could could be re-written in the following way: - if ( ret == XEN_TABLE_MAP_NONE ) - dprintk(XENLOG_WARNING, "Is va(%#lx) really mapped?\n", va); - else if ( ret == XEN_TABLE_SUPER_PAGE ) + if ( ret != XEN_TABLE_MAP_NONE ) pa = pte_to_paddr(*(table + offsets[level + 1]));
+ return pa;Don't you want to OR in the low 12 bits of VA here (unless "pa" is still 0)?
It is a bug, and IIUC if `pa` isn't 0 it is still need to add the low bits of
VA to `pa`:
return pa | (va & ((1 << PAGE_SHIFT) - 1));
(I think that I saw somewhere a macros to generate masks but can't find where)
Thanks.
~ Oleksii
